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/ConversationHeader.tsx

@@ -210,17 +210,14 @@ function ActiveCallBanner({ conversationId }: { conversationId: string }) {
   const justLeft = state.kind === 'idle' && lastCallConversationId === conversationId;
 
   // Once presence confirms the room is empty, drop the "just left" hint so the
-  // banner hides cleanly instead of sticking forever.
+  // banner hides cleanly instead of sticking forever. Grace window handles the
+  // brief gap between hangup and presence re-sync so we don't flicker.
   useEffect(() => {
     if (!justLeft) return;
-    if (active.length === 0) return;
-    if (othersIn.length === 0) {
-      // Active reports only us (or something stale) — wait, then dismiss.
-      const id = window.setTimeout(() => dismissLastCall(), 3000);
-      return () => window.clearTimeout(id);
-    }
-    return undefined;
-  }, [justLeft, active.length, othersIn.length, dismissLastCall]);
+    if (othersIn.length > 0) return; // still live — keep banner
+    const id = window.setTimeout(() => dismissLastCall(), 3000);
+    return () => window.clearTimeout(id);
+  }, [justLeft, othersIn.length, dismissLastCall]);
 
   if (iAmIn) return null;
   if (othersIn.length === 0 && !justLeft) return null;