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/app/methods/messages.rs

@@ -47,6 +47,7 @@ impl<T: TdClientTrait> MessageMethods<T> for App<T> {
         if let ChatState::MessageSelection { selected_index } = &mut self.chat_state {
             if *selected_index > 0 {
                 *selected_index -= 1;
+                self.stop_playback();
             }
         }
     }
@@ -59,9 +60,11 @@ impl<T: TdClientTrait> MessageMethods<T> for App<T> {
         if let ChatState::MessageSelection { selected_index } = &mut self.chat_state {
             if *selected_index < total - 1 {
                 *selected_index += 1;
+                self.stop_playback();
             } else {
                 // Дошли до самого нового сообщения - выходим из режима выбора
                 self.chat_state = ChatState::Normal;
+                self.stop_playback();
             }
         }
     }