feat(reco): Recommendations in all Discover flyouts + opt-out setting

Surface the new Recommendations tab everywhere Discover sub-sections are listed,
and make it opt-out.

- Flyouts: the sidebar section-nav flyout and both header nav flyouts (full text
  + compact icon) now list "Recommendations" alongside Home / Editor's Picks /
  For You. The Rust route handler already maps "discover-recommendations"
  (strip_prefix + select_tab + ensure_loaded), so no routing change was needed.
- Opt-out: a show_recommendations flag (default on) persisted in the per-user
  discover-prefs store (qbz_app), seeded into SettingsState on session start.
  Settings > Integrations gains a "Show Recommendations in Discover" toggle whose
  description nudges connecting Last.fm + ListenBrainz (MusicBrainz is used
  automatically to match releases). Off => the tab pill and every flyout entry
  disappear.
- Gating uses two mounted instances per flyout (one with the entry, one without)
  because Slint can't type-infer [NavMenuEntry] from a ternary of array literals;
  the HomeView pill uses a plain if.

cargo check -p qbz green.

cc

crates/qbz-ui/ui/discover/HomeView.slint

@@ -8,7 +8,7 @@
 
 import { Theme } from "../foundation/semantic-colors.slint";
 import { Typography } from "../foundation/typography.slint";
-import { HomeState, HomeActions, DiscoverState, DiscoverActions, GenreFilterState, GenreFilterActions, NavState, ContentView, ShellState } from "../state.slint";
+import { HomeState, HomeActions, DiscoverState, DiscoverActions, GenreFilterState, GenreFilterActions, NavState, ContentView, ShellState, SettingsState } from "../state.slint";
 import { Carousel } from "Carousel.slint";
 import { SlimCarousel } from "SlimCarousel.slint";
 import { PlaylistCarousel } from "PlaylistCarousel.slint";
@@ -211,7 +211,7 @@ export component HomeView inherits Rectangle {
                             active: HomeState.active-tab == "forYou";
                             clicked => { HomeActions.select-tab("forYou"); }
                         }
-                        Tab {
+                        if SettingsState.show-recommendations: Tab {
                             label: @tr("Recommendations");
                             active: HomeState.active-tab == "recommendations";
                             clicked => { HomeActions.select-tab("recommendations"); }