fix: lock viewport height to prevent topbar scroll-off

Body fixed to 100dvh with overflow:hidden. SOS button capped at
min(60vmin, 70vw, 300px) to fit within viewport alongside topbar.
Bump SW cache to v3.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/style.css

@@ -28,14 +28,14 @@ html, body {
   -webkit-tap-highlight-color: transparent;
   overscroll-behavior: none;
   user-select: none;
+  overflow: hidden;
 }
 
 body {
   display: flex;
   flex-direction: column;
-  min-height: 100vh;
-  /* Use dynamic viewport height on mobile to account for browser chrome */
-  min-height: 100dvh;
+  height: 100vh;
+  height: 100dvh;
 }
 
 /* ===== Private mode banner (decision #1041) ===== */
@@ -148,10 +148,8 @@ body {
 /* ===== SOS button (min 60vmin × 60vmin per UX spec) ===== */
 
 .btn-sos {
-  width: 60vmin;
-  height: 60vmin;
-  min-width: 180px;
-  min-height: 180px;
+  width: min(60vmin, 70vw, 300px);
+  height: min(60vmin, 70vw, 300px);
   border-radius: 50%;
   border: none;
   background: var(--sos);