Use relative API paths for Tailscale access

Replaced hardcoded http://localhost:8420/api with /api so the
frontend works from any host (Tailscale, LAN, etc).

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

web/frontend/src/views/ProjectView.vue

@@ -133,7 +133,7 @@ async function addDecision() {
       category: decForm.value.category || undefined,
       tags,
     }
-    const res = await fetch('http://localhost:8420/api/decisions', {
+    const res = await fetch('/api/decisions', {
       method: 'POST',
       headers: { 'Content-Type': 'application/json' },
       body: JSON.stringify(body),