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/audio/mod.rs

@@ -0,0 +1,11 @@
+//! Audio playback module for voice messages.
+//!
+//! Provides:
+//! - AudioPlayer: rodio-based playback with play/pause/stop/volume controls
+//! - VoiceCache: LRU cache for downloaded OGG voice files
+
+pub mod cache;
+pub mod player;
+
+pub use cache::VoiceCache;
+pub use player::AudioPlayer;