feat(web): add browser web client with IndexedDB, Service Worker, and PWA

Create sdks/web/ with a vanilla TypeScript web client:
- IndexedDB local store for conversations, messages, and identity
- WebSocket transport connecting to the server bridge
- Service Worker with cache-first strategy for offline support
- PWA manifest for installable web app
- Dark-themed responsive UI with sidebar, messages, and input bar
- Connection status badge and MLS epoch indicator in header
- Unread message count badges on conversations

sdks/web/public/manifest.json

@@ -0,0 +1,21 @@
+{
+  "name": "quicproquo",
+  "short_name": "qpq",
+  "description": "End-to-end encrypted group messenger",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#0d1117",
+  "theme_color": "#58a6ff",
+  "icons": [
+    {
+      "src": "icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png"
+    },
+    {
+      "src": "icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png"
+    }
+  ]
+}