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>

src/lib/actions.ts

@@ -1,4 +1,5 @@
-import { session } from './session.svelte.ts';
+import { session, sessions } from './session.svelte.ts';
+import { share } from './share.svelte.ts';
 import { ui } from './ui.svelte.ts';
 import { store } from './settings.svelte.ts';
 import { voice, userKey } from './audio/voice.svelte.ts';
@@ -67,6 +68,7 @@ export function userMenu(u: User): MenuItem[] {
     );
     if (u.userId == null) items.push({ label: 'Register on this server', action: () => session.register(u.session) });
   } else {
+    if (session.sharing[u.session]) items.push({ label: 'Watch screen', action: () => share.watch(sessions.active, u.session) });
     items.push(
       { label: 'Send message', action: () => session.selectView(`dm:${u.session}`) },
       { label: 'Mute for me', checked: !!voice.settings.localMutes[userKey(u)], action: () => voice.setLocalMute(u, !voice.settings.localMutes[userKey(u)]) },