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/cache.rs

@@ -0,0 +1,158 @@
+//! Voice message cache management.
+//!
+//! Caches downloaded OGG voice files in ~/.cache/tele-tui/voice/
+//! with LRU eviction when cache size exceeds limit.
+
+use std::collections::HashMap;
+use std::fs;
+use std::path::{Path, PathBuf};
+
+/// Maximum cache size in bytes (100 MB default)
+const MAX_CACHE_SIZE_BYTES: u64 = 100 * 1024 * 1024;
+
+/// Cache for voice message files
+pub struct VoiceCache {
+    cache_dir: PathBuf,
+    /// file_id -> (path, size_bytes, access_count)
+    files: HashMap<String, (PathBuf, u64, usize)>,
+    access_counter: usize,
+    max_size_bytes: u64,
+}
+
+impl VoiceCache {
+    /// Creates a new VoiceCache
+    pub fn new() -> Result<Self, String> {
+        let cache_dir = dirs::cache_dir()
+            .ok_or("Failed to get cache directory")?
+            .join("tele-tui")
+            .join("voice");
+
+        fs::create_dir_all(&cache_dir)
+            .map_err(|e| format!("Failed to create cache directory: {}", e))?;
+
+        Ok(Self {
+            cache_dir,
+            files: HashMap::new(),
+            access_counter: 0,
+            max_size_bytes: MAX_CACHE_SIZE_BYTES,
+        })
+    }
+
+    /// Gets the path for a cached voice file, if it exists
+    pub fn get(&mut self, file_id: &str) -> Option<PathBuf> {
+        if let Some((path, _, access)) = self.files.get_mut(file_id) {
+            // Update access count for LRU
+            self.access_counter += 1;
+            *access = self.access_counter;
+            Some(path.clone())
+        } else {
+            None
+        }
+    }
+
+    /// Stores a voice file in the cache
+    pub fn store(&mut self, file_id: &str, source_path: &Path) -> Result<PathBuf, String> {
+        // Copy file to cache
+        let filename = format!("{}.ogg", file_id.replace('/', "_"));
+        let dest_path = self.cache_dir.join(&filename);
+
+        fs::copy(source_path, &dest_path)
+            .map_err(|e| format!("Failed to copy voice file to cache: {}", e))?;
+
+        // Get file size
+        let size = fs::metadata(&dest_path)
+            .map_err(|e| format!("Failed to get file size: {}", e))?
+            .len();
+
+        // Store in cache
+        self.access_counter += 1;
+        self.files
+            .insert(file_id.to_string(), (dest_path.clone(), size, self.access_counter));
+
+        // Check if we need to evict
+        self.evict_if_needed()?;
+
+        Ok(dest_path)
+    }
+
+    /// Returns the total size of all cached files
+    pub fn total_size(&self) -> u64 {
+        self.files.values().map(|(_, size, _)| size).sum()
+    }
+
+    /// Evicts oldest files until cache is under max size
+    fn evict_if_needed(&mut self) -> Result<(), String> {
+        while self.total_size() > self.max_size_bytes && !self.files.is_empty() {
+            // Find least recently accessed file
+            let oldest_id = self
+                .files
+                .iter()
+                .min_by_key(|(_, (_, _, access))| access)
+                .map(|(id, _)| id.clone());
+
+            if let Some(id) = oldest_id {
+                self.evict(&id)?;
+            }
+        }
+        Ok(())
+    }
+
+    /// Evicts a specific file from cache
+    fn evict(&mut self, file_id: &str) -> Result<(), String> {
+        if let Some((path, _, _)) = self.files.remove(file_id) {
+            fs::remove_file(&path)
+                .map_err(|e| format!("Failed to remove cached file: {}", e))?;
+        }
+        Ok(())
+    }
+
+    /// Clears all cached files
+    pub fn clear(&mut self) -> Result<(), String> {
+        for (path, _, _) in self.files.values() {
+            let _ = fs::remove_file(path); // Ignore errors
+        }
+        self.files.clear();
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::io::Write;
+
+    #[test]
+    fn test_voice_cache_creation() {
+        let cache = VoiceCache::new();
+        assert!(cache.is_ok());
+    }
+
+    #[test]
+    fn test_cache_get_nonexistent() {
+        let mut cache = VoiceCache::new().unwrap();
+        assert!(cache.get("nonexistent").is_none());
+    }
+
+    #[test]
+    fn test_cache_store_and_get() {
+        let mut cache = VoiceCache::new().unwrap();
+
+        // Create temporary file
+        let temp_dir = std::env::temp_dir();
+        let temp_file = temp_dir.join("test_voice.ogg");
+        let mut file = fs::File::create(&temp_file).unwrap();
+        file.write_all(b"test audio data").unwrap();
+
+        // Store in cache
+        let result = cache.store("test123", &temp_file);
+        assert!(result.is_ok());
+
+        // Get from cache
+        let cached_path = cache.get("test123");
+        assert!(cached_path.is_some());
+        assert!(cached_path.unwrap().exists());
+
+        // Cleanup
+        fs::remove_file(&temp_file).unwrap();
+    }
+}