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/ui/Chat.svelte

@@ -3,6 +3,7 @@
   import ChatLog from './ChatLog.svelte';
   import Composer from './Composer.svelte';
   import VoiceStage from './VoiceStage.svelte';
+  import ShareView from './ShareView.svelte';
   import { ui } from '../lib/ui.svelte.ts';
   import { session } from '../lib/session.svelte.ts';
 
@@ -28,6 +29,7 @@
   </header>
 
   {#if showStage && session.status === 'connected'}<VoiceStage />{/if}
+  {#if session.status === 'connected'}<ShareView />{/if}
 
   {#if session.status === 'idle' && session.disconnectInfo}
     {@const d = session.disconnectInfo}