Camera sharing, and the proxy's relay for the desktop app

- A camera is a second source in the share dialog; streams carry their kind,
  so icons and wording follow. Browsers without screen capture (phones) start
  on the camera.
- Settings, Voice: the address of a mumh5 web version as relay; its STUN and
  relay are then offered with every stream and take over when no direct
  connection comes up.
- The proxy hands out relay credentials at GET /api/relay, readable from any
  origin.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/lib/icons.ts

@@ -6,6 +6,7 @@ export const ICON_PATHS: Record<string, string> = {
   'headphones-off': 'M3 14v-2a9 9 0 0 1 18 0v2M21 14v7h-4v-7zM3 14v7h4v-7zM3 3l18 18',
   settings: 'M4 21v-7M4 10V3M12 21v-9M12 8V3M20 21v-5M20 12V3M1 14h6M9 8h6M17 16h6',
   plus: 'M12 5v14M5 12h14',
+  camera: 'M2 6h14v12H2zM16 10l6-3v10l-6-3',
   screen: 'M3 4h18v12H3zM8 20h8M12 16v4',
   popout: 'M14 4h6v6M20 4l-9 9M18 14v6H4V6h6',
   expand: 'M4 9V4h5M20 9V4h-5M4 15v5h5M20 15v5h-5',