StubMusicPlaybackService is a base for test doubles with no production
caller, so `check-unused-code lib` flagged it and the analysis job
failed. Moves it to test/test_helpers/, where shared fakes belong.
Deduplicates the hand-rolled coalescing/caching maps, the Plex client cast,
the missing-serverId event guard and the progress-failure backoff, and drops
the MusicPlaybackService availability gate, which could never fail in
production.
- PaginatedCardGridTabState: the collections and playlists tabs were 95%
identical; they now supply only pageSize/fetchPage/idOf instead of each
duplicating the grid, memo, inflation budget and focus wiring.
- EtagCachedRemoteStore: the anime-lists and fribb mapping stores now share
one download/cache/isolate-parse/conditional-GET lifecycle.
- FocusableTileStateMixin manages its own initState/didUpdateWidget/dispose
instead of requiring every caller to forward three lifecycle hooks.
Also drops unused ServerCapabilities entries and dead code in
focusable_list_tile and music/track_row.
Queue sheet/panel now renders the whole playback order — dimmed played
tracks above the current one — instead of only up-next. Opens scrolled
to the current track and follows advancement while parked on it.
edde746