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/tsconfig.json

@@ -0,0 +1,17 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "ES2022",
+    "moduleResolution": "bundler",
+    "lib": ["ES2022", "DOM", "DOM.Iterable"],
+    "outDir": "dist",
+    "rootDir": "src",
+    "strict": true,
+    "sourceMap": true,
+    "declaration": false,
+    "esModuleInterop": true,
+    "skipLibCheck": true
+  },
+  "include": ["src/**/*.ts"],
+  "exclude": ["node_modules", "dist"]
+}