feat: complete Phase 12 — voice playback ticker, cache, config, and UI

Add playback position ticker in event loop with 1s UI refresh rate,
integrate VoiceCache for downloaded voice files, add [audio] config
section (cache_size_mb, auto_download_voice), and render progress bar
with waveform visualization in message bubbles.

Fix race conditions in AudioPlayer: add `starting` flag to prevent
false `is_stopped()` during ffplay startup, guard pid cleanup so old
threads don't overwrite newer process pids. Implement `resume_from()`
with ffplay `-ss` for real audio seek on unpause (-1s rewind).

Kill ffplay on app exit via `stop_playback()` in shutdown + Drop impl.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/audio/cache.rs

@@ -7,9 +7,6 @@ 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,
@@ -20,8 +17,8 @@ pub struct VoiceCache {
 }
 
 impl VoiceCache {
-    /// Creates a new VoiceCache
-    pub fn new() -> Result<Self, String> {
+    /// Creates a new VoiceCache with the given max size in MB
+    pub fn new(max_size_mb: u64) -> Result<Self, String> {
         let cache_dir = dirs::cache_dir()
             .ok_or("Failed to get cache directory")?
             .join("tele-tui")
@@ -34,7 +31,7 @@ impl VoiceCache {
             cache_dir,
             files: HashMap::new(),
             access_counter: 0,
-            max_size_bytes: MAX_CACHE_SIZE_BYTES,
+            max_size_bytes: max_size_mb * 1024 * 1024,
         })
     }
 
@@ -123,19 +120,19 @@ mod tests {
 
     #[test]
     fn test_voice_cache_creation() {
-        let cache = VoiceCache::new();
+        let cache = VoiceCache::new(100);
         assert!(cache.is_ok());
     }
 
     #[test]
     fn test_cache_get_nonexistent() {
-        let mut cache = VoiceCache::new().unwrap();
+        let mut cache = VoiceCache::new(100).unwrap();
         assert!(cache.get("nonexistent").is_none());
     }
 
     #[test]
     fn test_cache_store_and_get() {
-        let mut cache = VoiceCache::new().unwrap();
+        let mut cache = VoiceCache::new(100).unwrap();
 
         // Create temporary file
         let temp_dir = std::env::temp_dir();