feat(slint): Discover genre filter (simple view)

Ports Tauri's Filter-by-genre popup (simple grid view). The selection
is shared across the three Discover tabs — matching Tauri's single
"home" genre context — and re-fetches the discover index with the
chosen genre ids.

UI:
  - New GenreFilterPopup: header (icon + title + close), Remember
    selection + Advanced view toggles (QbzToggle), a 3-column grid of
    parent-genre chips with an accent fill + check circle when
    selected, and a Clear filter footer. Anchored top-right under the
    button, with a click-away backdrop.
  - GenreButton turns accent-filled and shows "N genres" / "1 genre"
    when a filter is active (else "Filter by genre"), 1:1 with Tauri.
  - Advanced view (search + 3-level tree + counts) is the next
    increment; its toggle is present and shows a "coming soon" stub.

State / backend:
  - GenreFilterState + GenreFilterActions globals; GenreChip struct.
  - crate::genre_filter loads the parent genres (QbzCore::get_genres)
    and owns the shared selection, persisted to
    <data-dir>/qbz/genre_filter.json when Remember is on (dropped
    when off). Stale ids are pruned against the loaded genres.
  - load_home now takes genre_ids and threads them into
    get_discover_index. The home-load path is extracted into a shared
    `reload_home(active_tab)` used by the initial shell load and by
    genre toggle/clear; it rebuilds all three section sets, restores
    the active tab via select_tab, and re-fires artwork.

cc

crates/qbz-slint/ui/app.slint

@@ -17,7 +17,7 @@ import { LoginScreen } from "login/LoginScreen.slint";
 import { AppShell } from "shell/AppShell.slint";
 
 // Re-export the state globals so the Rust layer can populate them.
-export { HomeState, HomeActions, AlbumState, ArtistState, NavState, SessionState, SettingsState, AlbumActions, ArtistActions, ArtworkActions, NowPlayingState, QueueState, SearchState, SearchActions, NetworkSidebarState, NetworkSidebarActions, MusicianState, MusicianActions, LabelState, LabelActions, LocationViewState, LocationViewActions, FavoritesState, FavoritesActions } from "state.slint";
+export { HomeState, HomeActions, GenreFilterState, GenreFilterActions, AlbumState, ArtistState, NavState, SessionState, SettingsState, AlbumActions, ArtistActions, ArtworkActions, NowPlayingState, QueueState, SearchState, SearchActions, NetworkSidebarState, NetworkSidebarActions, MusicianState, MusicianActions, LabelState, LabelActions, LocationViewState, LocationViewActions, FavoritesState, FavoritesActions } from "state.slint";
 
 // Which top-level screen is shown. The app starts on `splash` while it
 // restores a saved session, then resolves to `shell` or `login`.