feat: call UX overhaul — deafen sync, share dialog, fullscreen redesign

Speaking ring:
- Switch useActiveSpeakers from LiveKit's smoothed isSpeaking / server-
  batched ActiveSpeakersChanged to Web Audio API AnalyserNode on each
  participant's raw audio MediaStreamTrack. Poll 50ms, RMS threshold 0.03,
  250ms hold. Feels real-time vs the old ~500ms lag
- Defensive syncProbe on every tick so probes catch up if TrackPublished
  missed (local mic publish race on join)
- Universal speaking overlay on tile (3px emerald border + inset glow,
  z-10) so video mode shows the ring too, not just audio mode

Screen sharing:
- Separate "screen" tile per sharer so the sharer's avatar tile stays
  intact with its speaking ring. Tile.id is kind-prefixed (user:xxx /
  screen:xxx) so focus tracking distinguishes them
- New ScreenShareDialog (quality preset + fps override + displaySurface
  hint) opens on the share button. startScreenShare / stopScreenShare
  actions in CallContext replace the one-shot toggle
- ScreenShareViewer: plain CSS-only fullscreen overlay (Tauri WKWebView
  doesn't implement requestFullscreen), always `h-full w-full
  object-contain`, Esc exits

Camera:
- toggleCamera action in CallContext tracks isCameraEnabled
- VideoStub renders real <video> srcObject for the participant's camera
  MediaStreamTrack; local preview is mirrored
- Tile video track resolves to Track.Source.Camera publications of the
  LocalParticipant / each RemoteParticipant
- Room listens for TrackMuted / TrackUnmuted and re-publishes remote
  state so peers switch to avatar placeholder when a camera is disabled

Deafen:
- New isDeafened state + toggleDeafen action. Sets `muted = true` on all
  attached `<audio[data-livekit-track]>` plus mutes fresh ones on attach
  via module-level flag
- Broadcast state over the LiveKit data channel
  ({type:'presence', deafened}) so peers can render the headphones-off
  badge. Attributes API not used because the self-hosted server may run
  older LiveKit versions
- remoteDeafen: Record<identity, bool> exposed via context, bumped on
  DataReceived and re-broadcast on ParticipantConnected

Incoming video call:
- acceptIncoming takes an optional CallKind override so the receiver can
  answer a video invite with audio only or promote an audio invite to
  video on accept
- IncomingCallPanel shows two accept buttons (audio + video) when the
  invite is a video call

Audio devices:
- audioSettings adds inputDeviceId + outputDeviceId, persisted
- CallContext uses them on setMicrophoneEnabled, plus new
  setAudioInputDevice / setAudioOutputDevice hot-swap actions.
  Output swap applies setSinkId to every attached remote-audio element
  since LiveKit's own switchActiveDevice only tracks elements it
  attached itself
- SettingsPage "Mikrofon" + "Ausgabegerät" selects with devicechange
  listener and a permission-probe button

Fullscreen mode:
- Replaced absolute-positioned speaker + floating thumbnails with a real
  flex layout. Default = even grid of all tiles. Clicking a tile flips
  to big-speaker + horizontal thumbnail strip. Click focused tile =
  back to grid
- Controls overlay pinned bottom; content wrapper has pb-24 so tiles
  never sit behind the toolbar
- Grid now uses explicit grid-rows-* so cells get a defined 1fr height
  (without it, video intrinsic dimensions blew tiles past the container
  bounds on Windows)

UI chips:
- Mic-off badge combines isMuted flag AND
  localParticipant.isMicrophoneEnabled, so a user with no mic / denied
  permission sees the badge + the toolbar button red even though they
  never pressed mute
- Deafen badge on tile chips for local + remote (remote driven by the
  data-channel broadcast)

apps/desktop/src/components/CallControls.tsx

@@ -1,6 +1,8 @@
 import { useTranslation } from 'react-i18next';
 
 import {
+  HeadphonesIcon,
+  HeadphonesOffIcon,
   MicIcon,
   MicOffIcon,
   MonitorShareIcon,
@@ -14,9 +16,11 @@ interface Props {
   muted: boolean;
   sharing: boolean;
   video: boolean;
+  deafened: boolean;
   onToggleMute: () => void;
   onToggleShare: () => void;
   onToggleVideo?: () => void;
+  onToggleDeafen: () => void;
   onHangup: () => void;
   onOpenParticipants?: () => void;
   /** Compact variant used inside the docked call (36px buttons). */
@@ -31,9 +35,11 @@ export function CallControls({
   muted,
   sharing,
   video,
+  deafened,
   onToggleMute,
   onToggleShare,
   onToggleVideo,
+  onToggleDeafen,
   onHangup,
   onOpenParticipants,
   compact = false,
@@ -59,6 +65,24 @@ export function CallControls({
       >
         {muted ? <MicOffIcon className="h-5 w-5" /> : <MicIcon className="h-5 w-5" />}
       </CallButton>
+      <CallButton
+        label={
+          deafened
+            ? t('app:call.undeafen', { defaultValue: 'Ton wieder aktiv' })
+            : t('app:call.deafen', { defaultValue: 'Alle stumm' })
+        }
+        active={deafened}
+        activeTone="danger"
+        onClick={onToggleDeafen}
+        glass={glass}
+        className={btnSize}
+      >
+        {deafened ? (
+          <HeadphonesOffIcon className="h-5 w-5" />
+        ) : (
+          <HeadphonesIcon className="h-5 w-5" />
+        )}
+      </CallButton>
       {onToggleVideo && (
         <CallButton
           label={t('app:call.start_video', { defaultValue: 'Video' })}