Add standalone browser mode: no daemon required

BrowserAdapter speaks yaw/2.1 signaling + WebRTC protocol entirely in
the browser (libsodium-wrappers for Ed25519/X25519). Identity stored in
localStorage; peers connect via any waste/yaw2 anchor.

When served from a non-localhost origin the app defaults to browser
mode. On localhost the daemon adapter is tried first with a one-click
switch to browser mode. config.js (gitignored, served by anchor) lets
the host pre-inject signalURL and other defaults.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/package.json

@@ -10,6 +10,7 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "libsodium-wrappers": "^0.8.4",
     "react": "^19.2.6",
     "react-dom": "^19.2.6",
     "zustand": "^5.0.14"