Remove dead code and #![allow(dead_code)] blanket suppressions

- Delete src/audio/queue.rs (321 lines, PlayQueue never used)
- Remove #![allow(dead_code)] from audio, subsonic, and mpris module roots
- Remove unused MpvEvent2 enum, playlist_clear, get_volume, get_path,
  is_eof, observe_property from mpv.rs
- Remove unused DEFAULT_DEVICE_ID, is_available, get_effective_rate
  from pipewire.rs (and associated dead test)
- Remove unused search(), get_cover_art_url() from subsonic client
- Remove unused SearchResult3Data, SearchResult3 model structs
- Move parse_song_id_from_url into #[cfg(test)] block (only used by tests)
- Add targeted #[allow(dead_code)] on deserialization-only fields
  (MpvEvent, SubsonicResponseInner.version, ArtistIndex.name)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/mpris/mod.rs

@@ -1,5 +1,3 @@
 //! MPRIS2 D-Bus integration module
 
-#![allow(dead_code)]
-
 pub mod server;