Add screen sharing between mumh5 users (picture only)

- Signals travel as Mumble plugin data (compressed, chunked under Murmur's
  1000 byte limit); the picture goes directly to each viewer over WebRTC
- Toolbar button with a source picker in the desktop app, an indicator next to
  people who share, click to watch, a view above the chat with full screen
- Late joiners learn about running streams; moving, stopping or disconnecting
  ends them for viewers
- A note about IP addresses before the first use, optional STUN server setting
- Unit tests for the signal codec and an E2E with two app instances

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

package.json

@@ -20,6 +20,7 @@
     "dist:mac": "npm run build && electron-builder --mac",
     "test:e2e": "node test/e2e/app.e2e.ts",
     "test:e2e:web": "node test/e2e/web.e2e.ts",
+    "test:e2e:share": "node test/e2e/share.e2e.ts",
     "proto": "pbjs -t static-module -w es6 --keep-case --no-delimited --no-service --no-comments --force-number proto/Mumble.proto -o src/core/mumble-pb.js && pbjs -t static-module -w es6 --keep-case --no-delimited --no-service --no-comments --force-number proto/MumbleUDP.proto -o src/core/mumble-udp-pb.js",
     "screenshots": "node test/e2e/screenshots.ts"
   },