feat: audio devices + fullscreen + banner cleanup (v0.7.0)

Audio device selection:
- audioSettings: persisted inputDeviceId + outputDeviceId
- CallContext: uses stored input deviceId on mic enable, new
  setAudioInputDevice / setAudioOutputDevice actions that hot-swap
  without reconnect. Output swap applies HTMLMediaElement.setSinkId
  to every attached remote-audio element (LiveKit's switchActiveDevice
  only tracks elements it attached itself)
- SettingsPage: new "Mikrofon" + "Ausgabegerät" selects with
  enumerateDevices, devicechange listener, permission-probe button.
  setSinkId-unsupported fallback is messaged but non-blocking

Fullscreen:
- FullscreenCall was absolute inset-0 z-40 which trapped it inside the
  <main> pane — sidebar + chat-list stayed visible. Switched to
  fixed inset-0 z-[60] so the call overlays the whole window
  Discord-style
- ScreenShareViewer fullscreen: CSS-only toggle (native Fullscreen API
  unreliable under Tauri WKWebView), portalled to document.body when
  active so no ancestor stacking context can clip it. Esc exits

ActiveCallBanner:
- cleanup effect returned early when presence was entirely empty,
  leaving the "1 im Raum" fallback stuck after both peers left. Now
  schedules dismissLastCall as soon as othersIn.length === 0, with a
  3s grace window to absorb presence re-sync flicker

Bump tauri version 0.6.0 -> 0.7.0

apps/desktop/src/lib/audioSettings.ts

@@ -9,10 +9,19 @@ export type AudioQuality = 'voice' | 'hifi';
 
 export interface AudioSettings {
   quality: AudioQuality;
+  // Preferred input deviceId from enumerateDevices. null = use browser default
+  // (whatever the OS points at). Persisted across sessions, so "grandma's
+  // mic is default" stays even after browser picks the wrong device.
+  inputDeviceId: string | null;
+  // Preferred output (speaker/headphone) deviceId. null = system default.
+  // Applied to every <audio> element we attach via HTMLMediaElement.setSinkId.
+  outputDeviceId: string | null;
 }
 
 const DEFAULTS: AudioSettings = {
   quality: 'voice',
+  inputDeviceId: null,
+  outputDeviceId: null,
 };
 
 export interface AudioQualityParams {
@@ -73,6 +82,14 @@ function read(): AudioSettings {
     const parsed = JSON.parse(raw) as Partial<AudioSettings>;
     cached = {
       quality: isQuality(parsed.quality) ? parsed.quality : DEFAULTS.quality,
+      inputDeviceId:
+        typeof parsed.inputDeviceId === 'string' && parsed.inputDeviceId.length > 0
+          ? parsed.inputDeviceId
+          : DEFAULTS.inputDeviceId,
+      outputDeviceId:
+        typeof parsed.outputDeviceId === 'string' && parsed.outputDeviceId.length > 0
+          ? parsed.outputDeviceId
+          : DEFAULTS.outputDeviceId,
     };
     return cached;
   } catch {