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/app.slint

@@ -8,6 +8,9 @@ import { Theme } from "foundation/semantic-colors.slint";
 import { LoginScreen } from "login/LoginScreen.slint";
 import { AppShell } from "shell/AppShell.slint";
 
+// Re-export the HomeState global so the Rust layer can populate it.
+export { HomeState } from "state.slint";
+
 // Which top-level screen is shown. During M1 the screen switches on a
 // login-button click without real authentication; M2 ties the switch
 // to a real session.