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/package.json

@@ -0,0 +1,17 @@
+{
+  "name": "@quicproquo/web",
+  "version": "0.1.0",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "build": "tsc && cp -r public/* dist/",
+    "dev": "tsc --watch",
+    "clean": "rm -rf dist"
+  },
+  "dependencies": {
+    "@quicproquo/client": "file:../typescript"
+  },
+  "devDependencies": {
+    "typescript": "^5.0.0"
+  }
+}