feat: Conversation mode (ear button) - auto-record after ARIA speaks

- Ear button activates conversation mode (green dot)
- After TTS playback finishes → 800ms pause → auto-start recording
- VAD stops recording on silence → sends to ARIA → ARIA answers → TTS → loop
- Like a natural conversation / walkie-talkie mode
- Audio service fires onPlaybackFinished when queue empty

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

android/src/services/audio.ts

@@ -214,10 +214,22 @@ class AudioService {
     }
   }
 
+  // Callback wenn alle Audio-Teile abgespielt sind
+  private playbackFinishedListeners: (() => void)[] = [];
+
+  onPlaybackFinished(callback: () => void): () => void {
+    this.playbackFinishedListeners.push(callback);
+    return () => {
+      this.playbackFinishedListeners = this.playbackFinishedListeners.filter(cb => cb !== callback);
+    };
+  }
+
   /** Naechstes Audio aus der Queue abspielen */
   private async _playNext(): Promise<void> {
     if (this.audioQueue.length === 0) {
       this.isPlaying = false;
+      // Alle Audio-Teile abgespielt → Listener benachrichtigen
+      this.playbackFinishedListeners.forEach(cb => cb());
       return;
     }