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

tests/helpers/fake_tdclient_impl.rs

@@ -166,6 +166,11 @@ impl TdClientTrait for FakeTdClient {
         FakeTdClient::download_file(self, file_id).await
     }
 
+    async fn download_voice_note(&self, file_id: i32) -> Result<String, String> {
+        // Fake implementation: return a fake path
+        Ok(format!("/tmp/fake_voice_{}.ogg", file_id))
+    }
+
     // ============ Getters (immutable) ============
     fn client_id(&self) -> i32 {
         0 // Fake client ID