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/components/InCallPanel.tsx

@@ -487,7 +487,7 @@ function FullscreenCall({
   }, []);
 
   return (
-    <div className="absolute inset-0 z-40 flex flex-col overflow-hidden bg-surface">
+    <div className="fixed inset-0 z-[60] flex flex-col overflow-hidden bg-surface">
       <div className="relative min-h-0 flex-1">
         {speaker && (
           <div className="absolute inset-0">