fix: resolve all 40 clippy warnings (dead_code, unused_imports, lints)

- Add #[allow(unused_imports)] on pub re-exports used only by lib/tests
- Add #[allow(dead_code)] on public API items unused in binary target
- Fix collapsible_if, redundant_closure, unnecessary_map_or in main.rs
- Prefix unused test variables with underscore

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

src/audio/cache.rs

@@ -103,6 +103,7 @@ impl VoiceCache {
     }
 
     /// Clears all cached files
+    #[allow(dead_code)]
     pub fn clear(&mut self) -> Result<(), String> {
         for (path, _, _) in self.files.values() {
             let _ = fs::remove_file(path); // Ignore errors

src/audio/player.rs

@@ -139,11 +139,13 @@ impl AudioPlayer {
     }
 
     /// Returns true if a process is active (playing or paused)
+    #[allow(dead_code)]
     pub fn is_playing(&self) -> bool {
         self.current_pid.lock().unwrap().is_some() && !*self.paused.lock().unwrap()
     }
 
     /// Returns true if paused
+    #[allow(dead_code)]
     pub fn is_paused(&self) -> bool {
         self.current_pid.lock().unwrap().is_some() && *self.paused.lock().unwrap()
     }
@@ -153,13 +155,16 @@ impl AudioPlayer {
         self.current_pid.lock().unwrap().is_none() && !*self.starting.lock().unwrap()
     }
 
+    #[allow(dead_code)]
     pub fn set_volume(&self, _volume: f32) {}
+    #[allow(dead_code)]
     pub fn adjust_volume(&self, _delta: f32) {}
 
     pub fn volume(&self) -> f32 {
         1.0
     }
 
+    #[allow(dead_code)]
     pub fn seek(&self, _delta: Duration) -> Result<(), String> {
         Err("Seeking not supported".to_string())
     }