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/subsonic/models.rs

@@ -12,6 +12,7 @@ pub struct SubsonicResponse<T> {
 #[derive(Debug, Deserialize)]
 pub struct SubsonicResponseInner<T> {
     pub status: String,
+    #[allow(dead_code)] // Present in API response, needed for deserialization
     pub version: String,
     #[serde(default)]
     pub error: Option<ApiError>,
@@ -40,6 +41,7 @@ pub struct ArtistsIndex {
 
 #[derive(Debug, Deserialize)]
 pub struct ArtistIndex {
+    #[allow(dead_code)] // Present in API response, needed for deserialization
     pub name: String,
     #[serde(default)]
     pub artist: Vec<Artist>,
@@ -217,19 +219,3 @@ pub struct PlaylistDetail {
 #[derive(Debug, Deserialize)]
 pub struct PingData {}
 
-/// Search result
-#[derive(Debug, Deserialize)]
-pub struct SearchResult3Data {
-    #[serde(rename = "searchResult3")]
-    pub search_result3: SearchResult3,
-}
-
-#[derive(Debug, Deserialize)]
-pub struct SearchResult3 {
-    #[serde(default)]
-    pub artist: Vec<Artist>,
-    #[serde(default)]
-    pub album: Vec<Album>,
-    #[serde(default)]
-    pub song: Vec<Child>,
-}