feat(qbz-slint): real Discover Home view

After sign-in the app loads the real Qobuz discover index and renders
the Home view: a time-based greeting with the user's name, the tab
pill (Home / Editor's Picks / For You), and discover sections — New
Releases, Press Accolades, Ideal Discography, Qobuzissimes, Albums of
the Week — each a horizontally scrolling row of album cards.

- state.slint: AlbumCardItem / DiscoverSection structs, HomeState global.
- discover/: AlbumCard (artwork area, award ribbon, title, artist,
  quality badge), SectionRow, HomeView.
- home.rs: discover-index fetch and mapping (award-ribbon priority,
  quality string), greeting, and event-loop model application. Unit
  tests for ribbon, quality, and greeting logic.
- AppShell content region is now the HomeView.

Album artwork images load in the next increment (artwork pipeline);
cards currently show the elevated placeholder. No fixtures — all data
is the live discover index.

cc

crates/qbz-slint/ui/state.slint

@@ -0,0 +1,26 @@
+// Shared UI state — structs and globals the Rust controllers populate.
+//
+// The Rust side builds these models on a worker thread, then sets them
+// on the global from the Slint event loop (see src/home.rs, src/main.rs).
+
+export struct AlbumCardItem {
+    id: string,
+    title: string,
+    artist: string,
+    quality: string,
+    ribbon: string,
+    ribbon-kind: string,   // "press" | "qobuzissime" | "albumOfTheWeek" | ""
+    artwork-url: string,
+}
+
+export struct DiscoverSection {
+    title: string,
+    albums: [AlbumCardItem],
+}
+
+// Discover / Home screen state.
+export global HomeState {
+    in property <string> greeting: "";
+    in property <[DiscoverSection]> sections: [];
+    in property <bool> loading: false;
+}