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>

CHANGELOG.md

@@ -6,6 +6,7 @@ All notable changes to mumh5. Versions follow the `version` in `package.json`.
 
 ### Added
 
+- Screen sharing between mumh5 users in a channel (picture only so far): a toolbar button starts it, others click the indicator next to your name to watch. Streams go directly between clients; a STUN server can be set in Settings, Voice.
 - Browser version: `npm run build:web` builds the web app and a small self-hosted proxy that bridges browsers to Mumble servers on an allowlist. Identities are kept in the browser, voice goes through the TCP tunnel. See "Browser version" in the README.
 - Link previews in chat: title, description and image for web links (up to two per message). By default they are fetched by your f0ckm upload host, so the linked sites never see your IP address. Can be switched to "fetched by this computer" or off in Settings, Chat and files.
 - Voice tiles on small windows: when the window is too narrow for the member list, the people in your voice channel appear as tiles above the chat, light up while they talk, and keep mute and deafen at hand. The tiles can be collapsed.