- 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.
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