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/trait.rs

@@ -92,6 +92,7 @@ pub trait TdClientTrait: Send {
 
     // ============ File methods ============
     async fn download_file(&self, file_id: i32) -> Result<String, String>;
+    async fn download_voice_note(&self, file_id: i32) -> Result<String, String>;
 
     // ============ Getters (immutable) ============
     fn client_id(&self) -> i32;