feat: implement Phase 12 — voice message playback with ffplay

Add voice message playback infrastructure:
- AudioPlayer using ffplay subprocess with SIGSTOP/SIGCONT for pause/resume
- VoiceCache with LRU eviction (100 MB limit)
- TDLib integration: VoiceInfo, VoiceDownloadState, PlaybackState types
- download_voice_note() in TdClientTrait
- Keybindings: Space (play/pause), ←/→ (seek ±5s)
- Auto-stop playback on message navigation
- Remove debug_log module

src/tdlib/client_impl.rs

@@ -164,6 +164,11 @@ impl TdClientTrait for TdClient {
         self.download_file(file_id).await
     }
 
+    async fn download_voice_note(&self, file_id: i32) -> Result<String, String> {
+        // Voice notes use the same download mechanism as photos
+        self.download_file(file_id).await
+    }
+
     fn client_id(&self) -> i32 {
         self.client_id()
     }