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/ScreenShareDialog.tsx

@@ -0,0 +1,233 @@
+import { useState } from 'react';
+import { useTranslation } from 'react-i18next';
+
+import {
+  type DisplaySurfaceHint,
+  getPresetParams,
+  getScreenShareSettings,
+  PRESET_ORDER,
+  type ScreenSharePreset,
+} from '../lib/screenShareSettings';
+import {
+  MonitorShareIcon,
+  SpinnerIcon,
+  XIcon,
+} from './icons';
+
+interface Props {
+  open: boolean;
+  onClose: () => void;
+  onStart: (opts: {
+    preset: ScreenSharePreset;
+    displaySurface: DisplaySurfaceHint;
+    framerate: number | null;
+  }) => Promise<void>;
+}
+
+const FRAMERATE_OPTIONS: ReadonlyArray<{ value: number | null; label: string }> = [
+  { value: null, label: 'Preset-Standard' },
+  { value: 15, label: '15 fps' },
+  { value: 30, label: '30 fps' },
+  { value: 60, label: '60 fps' },
+];
+
+// Discord-style pre-share dialog. The OS still owns the final source picker
+// (browser/OS limitation — only Chrome/Edge plus a native plugin can enumerate
+// windows from JS), but we pre-filter with the `displaySurface` hint and lock
+// in quality + framerate up-front so the user doesn't have to re-open the
+// system picker to adjust them mid-call.
+export function ScreenShareDialog({ open, onClose, onStart }: Props) {
+  const { t } = useTranslation(['app']);
+  const initial = getScreenShareSettings();
+  const [surface, setSurface] = useState<DisplaySurfaceHint>(initial.displaySurface);
+  const [preset, setPreset] = useState<ScreenSharePreset>(initial.preset);
+  const [framerate, setFramerate] = useState<number | null>(initial.framerateOverride);
+  const [busy, setBusy] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+
+  if (!open) return null;
+
+  async function handleStart() {
+    setBusy(true);
+    setError(null);
+    try {
+      await onStart({ preset, displaySurface: surface, framerate });
+      onClose();
+    } catch (err: unknown) {
+      setError(err instanceof Error ? err.message : 'screenshare failed');
+    } finally {
+      setBusy(false);
+    }
+  }
+
+  const presetParams = getPresetParams(preset);
+  const effectiveFps = framerate ?? presetParams.framerate;
+
+  return (
+    <div
+      role="dialog"
+      aria-modal="true"
+      aria-label={t('app:call.share_dialog_title', { defaultValue: 'Bildschirm teilen' })}
+      className="fixed inset-0 z-50 flex items-center justify-center bg-black/50 p-4"
+      onClick={onClose}
+    >
+      <div
+        onClick={(e) => e.stopPropagation()}
+        className="flex w-full max-w-lg flex-col overflow-hidden rounded-2xl border border-line bg-surface-3 shadow-xl"
+      >
+        <header className="flex items-center justify-between border-b border-line px-5 py-3">
+          <div className="flex items-center gap-2">
+            <MonitorShareIcon className="h-4 w-4 text-accent" />
+            <h3 className="font-display text-sm font-semibold text-fg">
+              {t('app:call.share_dialog_title', { defaultValue: 'Bildschirm teilen' })}
+            </h3>
+          </div>
+          <button
+            type="button"
+            onClick={onClose}
+            aria-label="Close"
+            className="flex h-8 w-8 cursor-pointer items-center justify-center rounded-lg text-fg-muted transition hover:bg-surface-2 hover:text-fg"
+          >
+            <XIcon className="h-4 w-4" />
+          </button>
+        </header>
+
+        <div className="space-y-5 p-5">
+          <div>
+            <p className="mb-2 text-xs font-semibold uppercase tracking-wider text-fg-muted">
+              {t('app:call.share_surface', { defaultValue: 'Quelle' })}
+            </p>
+            <div className="grid grid-cols-3 gap-2">
+              <SurfaceOption
+                active={surface === null}
+                onClick={() => setSurface(null)}
+                label={t('app:call.share_any', { defaultValue: 'Alle anzeigen' })}
+                sub={t('app:call.share_any_sub', { defaultValue: 'Bildschirm + Fenster' })}
+              />
+              <SurfaceOption
+                active={surface === 'monitor'}
+                onClick={() => setSurface('monitor')}
+                label={t('app:call.share_monitor', { defaultValue: 'Bildschirm' })}
+                sub={t('app:call.share_monitor_sub', { defaultValue: 'Ganzer Monitor' })}
+              />
+              <SurfaceOption
+                active={surface === 'window'}
+                onClick={() => setSurface('window')}
+                label={t('app:call.share_window', { defaultValue: 'Fenster' })}
+                sub={t('app:call.share_window_sub', { defaultValue: 'Einzelnes Fenster' })}
+              />
+            </div>
+          </div>
+
+          <div>
+            <p className="mb-2 text-xs font-semibold uppercase tracking-wider text-fg-muted">
+              {t('app:call.share_quality', { defaultValue: 'Qualität' })}
+            </p>
+            <select
+              value={preset}
+              onChange={(e) => setPreset(e.target.value as ScreenSharePreset)}
+              className="w-full rounded-lg border border-line bg-surface-2 px-3 py-2 text-sm text-fg focus:border-accent focus:outline-none focus:ring-2 focus:ring-accent/30"
+            >
+              {PRESET_ORDER.map((p) => (
+                <option key={p} value={p}>
+                  {getPresetParams(p).label} · ≤ {getPresetParams(p).bitrateKbps} kbps
+                </option>
+              ))}
+            </select>
+          </div>
+
+          <div>
+            <p className="mb-2 text-xs font-semibold uppercase tracking-wider text-fg-muted">
+              {t('app:call.share_fps', { defaultValue: 'Bildrate' })}
+            </p>
+            <div className="flex flex-wrap gap-2">
+              {FRAMERATE_OPTIONS.map((opt) => (
+                <button
+                  key={String(opt.value)}
+                  type="button"
+                  onClick={() => setFramerate(opt.value)}
+                  className={
+                    'cursor-pointer rounded-lg border px-3 py-1.5 text-xs font-medium transition ' +
+                    (framerate === opt.value
+                      ? 'border-accent bg-accent/15 text-accent'
+                      : 'border-line bg-surface-2 text-fg hover:bg-surface')
+                  }
+                >
+                  {opt.label}
+                </button>
+              ))}
+            </div>
+            <p className="mt-2 text-[11px] text-fg-muted">
+              {t('app:call.share_fps_effective', {
+                defaultValue: 'Effektiv: {{fps}} fps',
+                fps: effectiveFps,
+              })}
+            </p>
+          </div>
+
+          {error && (
+            <p role="alert" className="text-sm text-rose-600 dark:text-rose-300">
+              {error}
+            </p>
+          )}
+
+          <p className="text-[11px] text-fg-muted">
+            {t('app:call.share_hint', {
+              defaultValue:
+                'Nach "Teilen starten" öffnet das Betriebssystem den Quellen-Picker. Qualität + Bildrate werden bereits angewendet.',
+            })}
+          </p>
+        </div>
+
+        <footer className="flex items-center justify-end gap-2 border-t border-line bg-surface-2 px-5 py-3">
+          <button
+            type="button"
+            onClick={onClose}
+            className="cursor-pointer rounded-md border border-line bg-surface-3 px-3 py-2 text-sm text-fg transition hover:bg-surface-2"
+          >
+            {t('app:friends.action_cancel', { defaultValue: 'Abbrechen' })}
+          </button>
+          <button
+            type="button"
+            onClick={() => void handleStart()}
+            disabled={busy}
+            className="inline-flex cursor-pointer items-center gap-1.5 rounded-md bg-accent px-3 py-2 text-sm font-semibold text-accent-fg transition hover:brightness-110 disabled:cursor-not-allowed disabled:opacity-60"
+          >
+            {busy && <SpinnerIcon className="h-4 w-4" />}
+            <span>
+              {t('app:call.share_start', { defaultValue: 'Teilen starten' })}
+            </span>
+          </button>
+        </footer>
+      </div>
+    </div>
+  );
+}
+
+function SurfaceOption({
+  active,
+  onClick,
+  label,
+  sub,
+}: {
+  active: boolean;
+  onClick: () => void;
+  label: string;
+  sub: string;
+}) {
+  return (
+    <button
+      type="button"
+      onClick={onClick}
+      className={
+        'flex flex-col items-start gap-0.5 rounded-lg border p-2.5 text-left transition ' +
+        (active
+          ? 'border-accent bg-accent/10 text-fg'
+          : 'border-line bg-surface-2 text-fg hover:bg-surface')
+      }
+    >
+      <span className="text-xs font-semibold">{label}</span>
+      <span className="text-[10px] text-fg-muted">{sub}</span>
+    </button>
+  );
+}