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/lib/screenShareSettings.ts

@@ -14,12 +14,22 @@ export type ScreenSharePreset =
   | '1440p60'
   | '4k60';
 
+// Browser getDisplayMedia `displaySurface` hint. The OS picker honours this
+// to pre-filter the source list (monitor = whole display, window = single
+// window). `null` leaves everything selectable.
+export type DisplaySurfaceHint = 'monitor' | 'window' | null;
+
 export interface ScreenShareSettings {
   preset: ScreenSharePreset;
+  displaySurface: DisplaySurfaceHint;
+  // User-chosen framerate. `null` falls back to preset's default.
+  framerateOverride: number | null;
 }
 
 const DEFAULTS: ScreenShareSettings = {
   preset: 'auto',
+  displaySurface: null,
+  framerateOverride: null,
 };
 
 export interface PresetParams {
@@ -103,6 +113,14 @@ function read(): ScreenShareSettings {
     const parsed = JSON.parse(raw) as Partial<ScreenShareSettings>;
     cached = {
       preset: isPreset(parsed.preset) ? parsed.preset : DEFAULTS.preset,
+      displaySurface:
+        parsed.displaySurface === 'monitor' || parsed.displaySurface === 'window'
+          ? parsed.displaySurface
+          : DEFAULTS.displaySurface,
+      framerateOverride:
+        typeof parsed.framerateOverride === 'number' && parsed.framerateOverride > 0
+          ? parsed.framerateOverride
+          : DEFAULTS.framerateOverride,
     };
     return cached;
   } catch {