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

src/thoughtsync/app.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import mimetypes
 import os
 import secrets
 from datetime import timedelta
@@ -18,6 +19,10 @@ from .settings_api import bp as settings_bp
 
 STATIC_DIR = os.path.join(os.path.dirname(__file__), "static")
 
+# `.webmanifest` isn't in every base image's mime map; register it so the PWA
+# manifest is served as application/manifest+json instead of octet-stream.
+mimetypes.add_type("application/manifest+json", ".webmanifest")
+
 
 def create_app() -> Quart:
     # static_folder=None: the SPA catch-all below owns static serving.