m4: PWA install — manifest, icons, service worker

Make ThoughtSync installable ("Add to Home Screen") without going
offline-first (the Android app is the real offline client, M5):

- web app manifest (name, icons incl. maskable + SVG, standalone, theme)
- generated PNG icon set + apple-touch-icon + favicon, from committed
  SVG sources (a linked-thoughts constellation on the brand tile)
- minimal service worker: installable shell only — caches just an
  offline fallback page, never the app shell / hashed assets / API, so
  data stays fresh and deploys never serve a stale shell
- register the SW in main.ts (progressive enhancement; failures ignored)
- index.html: manifest/icon links, apple-mobile meta, description
- backend: register the .webmanifest MIME type so it serves as
  application/manifest+json
- README: note that install needs a secure context (HTTPS/localhost)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/public/manifest.webmanifest

@@ -0,0 +1,18 @@
+{
+  "id": "/",
+  "name": "ThoughtSync",
+  "short_name": "ThoughtSync",
+  "description": "Capture a fleeting thought in a second — a self-hosted Keep-style board that grows into a lightweight second brain.",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "any",
+  "background_color": "#ffffff",
+  "theme_color": "#F5C518",
+  "icons": [
+    { "src": "/icon-192.png", "sizes": "192x192", "type": "image/png", "purpose": "any" },
+    { "src": "/icon-512.png", "sizes": "512x512", "type": "image/png", "purpose": "any" },
+    { "src": "/icon-maskable-512.png", "sizes": "512x512", "type": "image/png", "purpose": "maskable" },
+    { "src": "/icon.svg", "sizes": "any", "type": "image/svg+xml", "purpose": "any" }
+  ]
+}