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/IncomingCallPanel.tsx

@@ -4,7 +4,7 @@ import { useTranslation } from 'react-i18next';
 import { useCall } from '../context/CallContext';
 import { useFriendshipsContext } from '../context/FriendshipsContext';
 import { AvatarColorKey, colorKeyFor } from './CallParticipantTile';
-import { LockIcon, PhoneIcon, PhoneOffIcon } from './icons';
+import { LockIcon, PhoneIcon, PhoneOffIcon, VideoIcon } from './icons';
 
 interface Props {
   conversation: ConversationSummary;
@@ -99,23 +99,37 @@ export function IncomingCallPanel({ conversation }: Props) {
             </>
           )}
         </p>
-        <div className="mt-5 flex w-full max-w-[360px] gap-3">
+        <div className="mt-5 flex w-full max-w-[440px] flex-wrap gap-3">
           <button
             type="button"
             onClick={rejectIncoming}
-            className="inline-flex flex-1 cursor-pointer items-center justify-center gap-2 rounded-[14px] border border-rose-500/40 bg-transparent px-5 py-3.5 text-sm font-semibold text-rose-600 transition hover:bg-rose-500/10 focus:outline-none focus-visible:ring-2 focus-visible:ring-rose-500/40 dark:text-rose-400"
+            className="inline-flex flex-1 basis-[30%] cursor-pointer items-center justify-center gap-2 rounded-[14px] border border-rose-500/40 bg-transparent px-4 py-3.5 text-sm font-semibold text-rose-600 transition hover:bg-rose-500/10 focus:outline-none focus-visible:ring-2 focus-visible:ring-rose-500/40 dark:text-rose-400"
           >
             <PhoneOffIcon className="h-4 w-4" />
             <span>{t('app:call.decline')}</span>
           </button>
           <button
             type="button"
-            onClick={() => void acceptIncoming()}
-            className="inline-flex flex-1 cursor-pointer items-center justify-center gap-2 rounded-[14px] bg-emerald-600 px-5 py-3.5 text-sm font-semibold text-white shadow-accept-btn transition hover:bg-emerald-500 focus:outline-none focus-visible:ring-2 focus-visible:ring-emerald-500/50"
+            onClick={() => void acceptIncoming('audio')}
+            className="inline-flex flex-1 basis-[30%] cursor-pointer items-center justify-center gap-2 rounded-[14px] bg-emerald-600 px-4 py-3.5 text-sm font-semibold text-white shadow-accept-btn transition hover:bg-emerald-500 focus:outline-none focus-visible:ring-2 focus-visible:ring-emerald-500/50"
           >
             <PhoneIcon className="h-4 w-4" />
-            <span>{t('app:call.accept')}</span>
+            <span>
+              {state.mediaKind === 'video'
+                ? t('app:call.accept_audio', { defaultValue: 'Nur Audio' })
+                : t('app:call.accept')}
+            </span>
           </button>
+          {state.mediaKind === 'video' && (
+            <button
+              type="button"
+              onClick={() => void acceptIncoming('video')}
+              className="inline-flex flex-1 basis-[30%] cursor-pointer items-center justify-center gap-2 rounded-[14px] bg-accent px-4 py-3.5 text-sm font-semibold text-accent-fg shadow-accept-btn transition hover:brightness-110 focus:outline-none focus-visible:ring-2 focus-visible:ring-accent/50"
+            >
+              <VideoIcon className="h-4 w-4" />
+              <span>{t('app:call.accept_video', { defaultValue: 'Mit Video' })}</span>
+            </button>
+          )}
         </div>
       </div>
     </section>