Make genre_filter.rs selection per-context (HashMap keyed by "discover" /
"favorites" + a current context the popup edits), so favorites filters
independently from Discover. Discover keeps its server-side path
unchanged (reads filter_ids("discover") explicitly; its genre buttons set
the "discover" context on open). Favorites get a genre button in both
toolbars that opens the shared popup in the "favorites" context, plus the
popup overlay; derive_tracks / derive_albums filter client-side by the
selected genre NAMES (selected_names expands parents to descendants).
on_toggle / on_clear branch on the current context to re-derive favorites
instead of re-fetching Discover. AlbumSummary gains an optional `genre`
(favorites track album objects carry it; TrackItem can't, so a track
id->genre map backs the tracks filter). Persisted per-context (legacy
flat selection migrates to "discover").
cc
vicrodh
4dddd36073