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/config.example.js

@@ -0,0 +1,7 @@
+// Copy to config.js (gitignored) on the anchor host.
+// Served at /config.js; loaded before the React bundle in index.html.
+window.WASTE_CONFIG = {
+  signalURL: 'wss://waste.dev.xplwd.com/ws',
+  // Uncomment to pre-fill the network name on this invite URL:
+  // defaultNetwork: 'friends',
+}