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/ui.svelte.ts

@@ -9,6 +9,8 @@ export interface PromptRequest {
   initial?: string;
   text?: string;
   onsubmit: (value: string) => void;
+  // Closed without confirming
+  oncancel?: () => void;
 }
 
 export interface CertRequest {