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/state.slint

@@ -56,6 +56,33 @@ export global HomeActions {
     callback select-tab(string);
 }
 
+// One genre in the Filter-by-genre popup's simple grid.
+export struct GenreChip {
+    id: string,
+    name: string,
+    selected: bool,
+}
+
+// Filter-by-genre state. The selection is shared across the three
+// Discover tabs (matching Tauri's single "home" genre context) — it
+// re-fetches the discover index with the chosen genre ids. Populated
+// by crate::genre_filter.
+export global GenreFilterState {
+    in property <[GenreChip]> genres: [];
+    in property <int> selected-count: 0;
+    in-out property <bool> open: false;
+    in-out property <bool> remember: true;
+    in-out property <bool> advanced: false;
+    in property <bool> loading: false;
+}
+
+export global GenreFilterActions {
+    callback toggle(string /* genre id */);
+    callback clear();
+    callback set-remember(bool);
+    callback set-advanced(bool);
+}
+
 export struct AlbumTrackItem {
     id: string,
     number: string,