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>

electron/preload.ts

@@ -21,6 +21,8 @@ contextBridge.exposeInMainWorld('mumh5Native', {
   onContextMenu: (fn: (params: unknown) => void) => { ipcRenderer.on('context-menu', (_e, p) => fn(p)); },
   editAction: (action: string, arg?: unknown) => ipcRenderer.send('edit:action', action, arg),
   onTrayAction: (fn: (action: string) => void) => { ipcRenderer.on('tray:action', (_e, a: string) => fn(a)); },
+  screenSources: () => ipcRenderer.invoke('share:sources'),
+  pickScreenSource: (id: string) => ipcRenderer.send('share:pick', id),
   describeCerts: (ders: Uint8Array[]) => ipcRenderer.invoke('certs:describe', ders),
   identities: {
     list: () => ipcRenderer.invoke('identities:list'),