diff --git a/frontend/app.js b/frontend/app.js
new file mode 100644
index 0000000..10c4b1b
--- /dev/null
+++ b/frontend/app.js
@@ -0,0 +1,264 @@
+'use strict';
+
+// ========== Storage abstraction (decisions #1024, #1025) ==========
+//
+// #1024: probe availability with a real write, NOT typeof/in
+// #1025: fallback chain: localStorage → sessionStorage → in-memory
+
+let _storage = null;
+let _storageType = 'memory';
+const _mem = {};
+
+function _probeStorage(s) {
+  try {
+    const k = '__baton_probe__';
+    s.setItem(k, '1');
+    s.removeItem(k);
+    return true;
+  } catch (_) {
+    return false;
+  }
+}
+
+function _initStorage() {
+  if (_probeStorage(localStorage)) {
+    _storage = localStorage;
+    _storageType = 'local';
+  } else if (_probeStorage(sessionStorage)) {
+    _storage = sessionStorage;
+    _storageType = 'session';
+  } else {
+    _storage = {
+      getItem: (k) => (Object.prototype.hasOwnProperty.call(_mem, k) ? _mem[k] : null),
+      setItem: (k, v) => { _mem[k] = String(v); },
+      removeItem: (k) => { delete _mem[k]; },
+    };
+    _storageType = 'memory';
+  }
+}
+
+// ========== User identity ==========
+
+function _getOrCreateUserId() {
+  let id = _storage.getItem('baton_user_id');
+  if (!id) {
+    id = crypto.randomUUID();
+    _storage.setItem('baton_user_id', id);
+  }
+  return id;
+}
+
+function _isRegistered() {
+  return _storage.getItem('baton_registered') === '1';
+}
+
+function _getUserName() {
+  return _storage.getItem('baton_user_name') || '';
+}
+
+function _saveRegistration(name) {
+  _storage.setItem('baton_user_name', name);
+  _storage.setItem('baton_registered', '1');
+}
+
+function _getInitials(name) {
+  const trimmed = (name || '').trim();
+  if (!trimmed) return '?';
+  return trimmed
+    .split(/\s+/)
+    .map((w) => w[0] || '')
+    .join('')
+    .toUpperCase()
+    .slice(0, 2);
+}
+
+// ========== UI helpers ==========
+
+function _showScreen(id) {
+  document.querySelectorAll('.screen').forEach((el) => {
+    el.hidden = el.id !== id;
+  });
+}
+
+function _setStatus(msg, cls) {
+  const el = document.getElementById('status');
+  el.textContent = msg;
+  el.className = 'status' + (cls ? ' status--' + cls : '');
+  el.hidden = !msg;
+}
+
+function _updateNetworkIndicator() {
+  const el = document.getElementById('indicator-network');
+  if (!el) return;
+  el.className = 'network-indicator ' + (navigator.onLine ? 'online' : 'offline');
+  el.setAttribute('aria-label', navigator.onLine ? 'Online' : 'Offline');
+}
+
+function _updateUserAvatar() {
+  const el = document.getElementById('user-avatar');
+  if (!el) return;
+  el.textContent = _getInitials(_getUserName());
+}
+
+// ========== API calls ==========
+
+async function _apiPost(path, body) {
+  const res = await fetch(path, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify(body),
+  });
+  if (!res.ok) {
+    const text = await res.text().catch(() => '');
+    throw new Error('HTTP ' + res.status + (text ? ': ' + text : ''));
+  }
+  return res.json();
+}
+
+// ========== Geolocation (optional, non-blocking) ==========
+
+function _getGeo() {
+  return new Promise((resolve) => {
+    if (!navigator.geolocation) { resolve(null); return; }
+    navigator.geolocation.getCurrentPosition(
+      ({ coords }) =>
+        resolve({
+          lat: coords.latitude,
+          lon: coords.longitude,
+          accuracy: coords.accuracy,
+        }),
+      () => resolve(null),
+      { timeout: 5000, maximumAge: 30000 }
+    );
+  });
+}
+
+// ========== Handlers ==========
+
+async function _handleRegister() {
+  const input = document.getElementById('name-input');
+  const btn = document.getElementById('btn-confirm');
+  const name = input.value.trim();
+  if (!name) return;
+
+  btn.disabled = true;
+  _setStatus('', '');
+
+  try {
+    const uuid = _getOrCreateUserId();
+    await _apiPost('/api/register', { uuid, name });
+    _saveRegistration(name);
+    _updateUserAvatar();
+    _showMain();
+  } catch (_) {
+    _setStatus('Error. Please try again.', 'error');
+    btn.disabled = false;
+  }
+}
+
+function _setSosState(state) {
+  const btn = document.getElementById('btn-sos');
+  if (!btn) return;
+  btn.dataset.state = state;
+  btn.disabled = state === 'sending';
+}
+
+async function _handleSignal() {
+  // v1: no offline queue — show error and return (decision #1019)
+  if (!navigator.onLine) {
+    _setStatus('No connection. Check your network and try again.', 'error');
+    return;
+  }
+
+  _setSosState('sending');
+  _setStatus('', '');
+
+  try {
+    const geo = await _getGeo();
+    const uuid = _getOrCreateUserId();
+    const body = { user_id: uuid, timestamp: Date.now() };
+    if (geo) body.geo = geo;
+
+    await _apiPost('/api/signal', body);
+
+    _setSosState('success');
+    _setStatus('Signal sent!', 'success');
+    setTimeout(() => {
+      _setSosState('default');
+      _setStatus('', '');
+    }, 2000);
+  } catch (_) {
+    _setSosState('default');
+    _setStatus('Error sending. Try again.', 'error');
+  }
+}
+
+// ========== Screens ==========
+
+function _showOnboarding() {
+  _showScreen('screen-onboarding');
+
+  const input = document.getElementById('name-input');
+  const btn = document.getElementById('btn-confirm');
+
+  input.addEventListener('input', () => {
+    btn.disabled = input.value.trim().length === 0;
+  });
+  input.addEventListener('keydown', (e) => {
+    if (e.key === 'Enter' && !btn.disabled) _handleRegister();
+  });
+  btn.addEventListener('click', _handleRegister);
+}
+
+function _showMain() {
+  _showScreen('screen-main');
+  _updateUserAvatar();
+
+  const btn = document.getElementById('btn-sos');
+  if (btn && !btn.dataset.listenerAttached) {
+    btn.addEventListener('click', _handleSignal);
+    btn.dataset.listenerAttached = '1';
+  }
+}
+
+// ========== Service Worker ==========
+
+function _registerSW() {
+  if (!('serviceWorker' in navigator)) return;
+  navigator.serviceWorker.register('/sw.js').catch((err) => {
+    console.warn('[baton] SW registration failed:', err);
+  });
+}
+
+// ========== Init ==========
+
+function _init() {
+  _initStorage();
+
+  // Pre-generate and persist UUID on first visit (per arch spec flow)
+  _getOrCreateUserId();
+
+  // Private mode graceful degradation (decision #1041):
+  // show inline banner with explicit action guidance when localStorage is unavailable
+  if (_storageType !== 'local') {
+    const banner = document.getElementById('private-mode-banner');
+    if (banner) banner.hidden = false;
+  }
+
+  // Network indicator — initial state + live updates
+  _updateNetworkIndicator();
+  window.addEventListener('online', _updateNetworkIndicator);
+  window.addEventListener('offline', _updateNetworkIndicator);
+
+  // Route to correct screen
+  if (_isRegistered()) {
+    _showMain();
+  } else {
+    _showOnboarding();
+  }
+}
+
+document.addEventListener('DOMContentLoaded', () => {
+  _registerSW();
+  _init();
+});
diff --git a/frontend/icons/icon-192.png b/frontend/icons/icon-192.png
new file mode 100644
index 0000000..0ddef34
Binary files /dev/null and b/frontend/icons/icon-192.png differ
diff --git a/frontend/icons/icon-512.png b/frontend/icons/icon-512.png
new file mode 100644
index 0000000..7c2d680
Binary files /dev/null and b/frontend/icons/icon-512.png differ
diff --git a/frontend/index.html b/frontend/index.html
new file mode 100644
index 0000000..e5fe30e
--- /dev/null
+++ b/frontend/index.html
@@ -0,0 +1,78 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+  <meta charset="UTF-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
+  <meta name="description" content="Emergency signal button">
+
+  <!-- PWA meta tags -->
+  <meta name="theme-color" content="#ff0000">
+  <meta name="mobile-web-app-capable" content="yes">
+  <meta name="apple-mobile-web-app-capable" content="yes">
+  <meta name="apple-mobile-web-app-status-bar-style" content="black-translucent">
+  <meta name="apple-mobile-web-app-title" content="Baton">
+
+  <link rel="manifest" href="/manifest.json">
+  <link rel="apple-touch-icon" href="/icons/icon-192.png">
+  <link rel="stylesheet" href="/style.css">
+
+  <title>Baton</title>
+</head>
+<body>
+
+  <!-- Private mode banner: shown when localStorage is unavailable (decision #1041).
+       Explicit action guidance: user must open in normal mode to retain registration. -->
+  <div id="private-mode-banner" class="private-banner" hidden>
+    &#9888;&#65039; Private mode detected. Your registration will be lost when this tab
+    closes. To keep your data, open Baton in a regular (non-private) browser tab.
+  </div>
+
+  <!-- Shared top bar: user initials avatar + network status indicator -->
+  <header class="topbar">
+    <div class="user-avatar" id="user-avatar" aria-label="User">?</div>
+    <div class="network-indicator" id="indicator-network"
+         role="status" aria-label="Network status"></div>
+  </header>
+
+  <!-- Onboarding screen: shown on first visit (no UUID registered yet) -->
+  <div id="screen-onboarding" class="screen" role="main" hidden>
+    <div class="screen-content">
+      <input
+        type="text"
+        id="name-input"
+        class="name-input"
+        placeholder="Your name"
+        maxlength="100"
+        autocomplete="name"
+        autocorrect="off"
+        autocapitalize="words"
+        spellcheck="false"
+        aria-label="Your name"
+      >
+      <button type="button" id="btn-confirm" class="btn-confirm" disabled>
+        Confirm
+      </button>
+    </div>
+  </div>
+
+  <!-- Main screen: SOS button -->
+  <div id="screen-main" class="screen" role="main" hidden>
+    <div class="screen-content">
+      <button
+        type="button"
+        id="btn-sos"
+        class="btn-sos"
+        data-state="default"
+        aria-label="Send emergency signal"
+      >
+        SOS
+      </button>
+    </div>
+  </div>
+
+  <!-- Status message: errors and confirmations -->
+  <div id="status" class="status" role="alert" aria-live="polite" hidden></div>
+
+  <script src="/app.js"></script>
+</body>
+</html>
diff --git a/frontend/manifest.json b/frontend/manifest.json
new file mode 100644
index 0000000..9655cea
--- /dev/null
+++ b/frontend/manifest.json
@@ -0,0 +1,23 @@
+{
+  "name": "Baton",
+  "short_name": "Baton",
+  "description": "Emergency signal button",
+  "display": "standalone",
+  "orientation": "portrait-primary",
+  "start_url": "/",
+  "background_color": "#000000",
+  "theme_color": "#ff0000",
+  "icons": [
+    {
+      "src": "/icons/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png"
+    },
+    {
+      "src": "/icons/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any maskable"
+    }
+  ]
+}
diff --git a/frontend/style.css b/frontend/style.css
new file mode 100644
index 0000000..487a443
--- /dev/null
+++ b/frontend/style.css
@@ -0,0 +1,200 @@
+*, *::before, *::after {
+  box-sizing: border-box;
+  margin: 0;
+  padding: 0;
+}
+
+:root {
+  --bg:             #000000;
+  --text:           #ffffff;
+  --muted:          #9ca3af;
+  --input-bg:       #1a1a1a;
+  --border:         #374151;
+  --border-focus:   #6b7280;
+  --confirm-bg:     #374151;
+  --confirm-active: #4b5563;
+  --sos:            #cc2626;
+  --sos-pressed:    #991c1c;
+  --sos-success:    #16a34a;
+  --banner-bg:      #7c2d12;
+  --banner-text:    #fed7aa;
+}
+
+html, body {
+  height: 100%;
+  background: var(--bg);
+  color: var(--text);
+  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', system-ui, sans-serif;
+  -webkit-tap-highlight-color: transparent;
+  overscroll-behavior: none;
+  user-select: none;
+}
+
+body {
+  display: flex;
+  flex-direction: column;
+  min-height: 100vh;
+  /* Use dynamic viewport height on mobile to account for browser chrome */
+  min-height: 100dvh;
+}
+
+/* ===== Private mode banner (decision #1041) ===== */
+
+.private-banner {
+  background: var(--banner-bg);
+  color: var(--banner-text);
+  padding: 10px 16px;
+  font-size: 13px;
+  line-height: 1.5;
+  text-align: center;
+  flex-shrink: 0;
+}
+
+.private-banner[hidden] { display: none; }
+
+/* ===== Top bar ===== */
+
+.topbar {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  padding: 16px 20px;
+  flex-shrink: 0;
+}
+
+.user-avatar {
+  width: 36px;
+  height: 36px;
+  border-radius: 50%;
+  background: #374151;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  font-size: 13px;
+  font-weight: 600;
+  color: var(--text);
+  letter-spacing: 0;
+}
+
+.network-indicator {
+  width: 22px;
+  height: 22px;
+  border-radius: 50%;
+  background: var(--muted);
+  transition: background 0.3s ease;
+}
+
+.network-indicator.online  { background: #16a34a; }
+.network-indicator.offline { background: #4b5563; }
+
+/* ===== Screens ===== */
+
+.screen {
+  flex: 1;
+  display: flex;
+  flex-direction: column;
+  min-height: 0;
+}
+
+.screen[hidden] { display: none; }
+
+.screen-content {
+  flex: 1;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  gap: 16px;
+  padding: 24px 20px;
+}
+
+/* ===== Onboarding ===== */
+
+.name-input {
+  width: 100%;
+  max-width: 320px;
+  padding: 14px 16px;
+  background: var(--input-bg);
+  border: 1.5px solid var(--border);
+  border-radius: 12px;
+  color: var(--text);
+  font-size: 17px;
+  outline: none;
+  transition: border-color 0.15s;
+  -webkit-appearance: none;
+  user-select: text;
+}
+
+.name-input::placeholder { color: var(--muted); }
+.name-input:focus         { border-color: var(--border-focus); }
+
+.btn-confirm {
+  width: 100%;
+  max-width: 320px;
+  padding: 14px;
+  background: var(--confirm-bg);
+  border: none;
+  border-radius: 12px;
+  color: var(--text);
+  font-size: 17px;
+  font-weight: 600;
+  cursor: pointer;
+  transition: background 0.15s, opacity 0.15s;
+}
+
+.btn-confirm:disabled               { opacity: 0.35; cursor: default; }
+.btn-confirm:not(:disabled):active  { background: var(--confirm-active); }
+
+/* ===== SOS button (min 60vmin × 60vmin per UX spec) ===== */
+
+.btn-sos {
+  width: 60vmin;
+  height: 60vmin;
+  min-width: 180px;
+  min-height: 180px;
+  border-radius: 50%;
+  border: none;
+  background: var(--sos);
+  color: #fff;
+  font-size: clamp(24px, 9vmin, 60px);
+  font-weight: 900;
+  letter-spacing: 0.08em;
+  cursor: pointer;
+  touch-action: manipulation;
+  transition: background 0.15s ease, transform 0.1s ease;
+}
+
+.btn-sos:active,
+.btn-sos[data-state="sending"] {
+  background: var(--sos-pressed);
+  transform: scale(0.96);
+}
+
+.btn-sos[data-state="sending"] {
+  animation: baton-pulse 1s ease-in-out infinite;
+  cursor: default;
+  pointer-events: none;
+}
+
+.btn-sos[data-state="success"] {
+  background: var(--sos-success);
+  transform: none;
+}
+
+@keyframes baton-pulse {
+  0%, 100% { opacity: 1; }
+  50%       { opacity: 0.55; }
+}
+
+/* ===== Status message ===== */
+
+.status {
+  padding: 12px 20px;
+  text-align: center;
+  font-size: 14px;
+  flex-shrink: 0;
+}
+
+.status[hidden]    { display: none; }
+.status--error     { color: #f87171; }
+.status--success   { color: #4ade80; }
diff --git a/frontend/sw.js b/frontend/sw.js
new file mode 100644
index 0000000..e37d2fa
--- /dev/null
+++ b/frontend/sw.js
@@ -0,0 +1,60 @@
+'use strict';
+
+const CACHE_NAME = 'baton-v1';
+
+// App shell assets to precache
+const APP_SHELL = [
+  '/',
+  '/index.html',
+  '/app.js',
+  '/style.css',
+  '/manifest.json',
+  '/icons/icon-192.png',
+  '/icons/icon-512.png',
+];
+
+// Install: precache app shell + skipWaiting so new SW activates immediately
+self.addEventListener('install', (event) => {
+  self.skipWaiting();
+  event.waitUntil(
+    caches.open(CACHE_NAME).then((cache) => cache.addAll(APP_SHELL))
+  );
+});
+
+// Activate: delete stale caches + claim open clients (decision: skipWaiting + clients.claim)
+self.addEventListener('activate', (event) => {
+  event.waitUntil(
+    caches
+      .keys()
+      .then((keys) =>
+        Promise.all(
+          keys.filter((k) => k !== CACHE_NAME).map((k) => caches.delete(k))
+        )
+      )
+      .then(() => self.clients.claim())
+  );
+});
+
+// Fetch: cache-first for app shell; API calls pass through to network
+self.addEventListener('fetch', (event) => {
+  if (event.request.method !== 'GET') return;
+
+  const url = new URL(event.request.url);
+
+  // Never intercept API calls — they must always reach the server
+  if (url.pathname.startsWith('/api/')) return;
+
+  event.respondWith(
+    caches.match(event.request).then((cached) => {
+      if (cached) return cached;
+
+      return fetch(event.request).then((response) => {
+        if (response && response.status === 200) {
+          const clone = response.clone();
+          caches.open(CACHE_NAME).then((cache) => cache.put(event.request, clone));
+        }
+        return response;
+      });
+    })
+  );
+});