chore: senior-review cleanup pass

- Remove Gemini's committed update_*.py regex-surgery scripts
- Drop unused iroh-tickets dependency (hand-rolled ticket is used instead)
- Replace ToString antipattern with Display impl on PeerSpeakTicket
- Route debug log to XDG state/cache dir instead of hardcoded /home path
- Clear all compiler + clippy warnings (unused imports, collapsible ifs,
  redundant pattern matching, missing Default)

Builds clean with zero warnings.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/audio/pipewire_impl.rs

@@ -24,6 +24,12 @@ struct PlaybackState {
     thread: JoinHandle<()>,
 }
 
+impl Default for PipeWireBackend {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl PipeWireBackend {
     pub fn new() -> Self {
         pw::init();