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/ui/Sidebar.svelte

@@ -168,9 +168,9 @@
       onclick={() => session.setSelfDeaf(!deafened)} oncontextmenu={e => openQuick(e, 'output')}>
       <Icon name={deafened ? 'headphones-off' : 'headphones'} size={18} />
     </button>
-    <button class="icon-btn" class:live={sharingHere} title={sharingHere ? 'Stop sharing your screen' : 'Share your screen with your channel'}
-      aria-label={sharingHere ? 'Stop sharing your screen' : 'Share your screen'} disabled={!sharingHere && !canShare} onclick={toggleShare}>
-      <Icon name="screen" size={18} />
+    <button class="icon-btn" class:live={sharingHere} title={sharingHere ? `Stop sharing your ${share.kind}` : 'Share your screen or camera with your channel'}
+      aria-label={sharingHere ? `Stop sharing your ${share.kind}` : 'Share your screen or camera'} disabled={!sharingHere && !canShare} onclick={toggleShare}>
+      <Icon name={sharingHere ? share.kind : 'screen'} size={18} />
     </button>
     <span class="sep"></span>
     <button class="icon-btn" title="Change your description" aria-label="Change your description" disabled={!self} onclick={editDescription}>