feat(slint): LabelReleasesView (Port 3, second of three click targets)

Wires the artist network sidebar's Labels section to a real
destination. Mirrors Tauri's LabelReleasesView.svelte core: a label
header (image + name + release count) over a wrapping album grid
with load-more pagination. (Sort / filter Hi-Res / group-by-artist
from the Tauri view are deferred — the core browse experience is in.)

Slint:
  - ContentView gains `label`; NavEntry mirrors with
    `Label { id, name }`.
  - LabelState / LabelActions globals carry the header (id, name,
    image-url + decoded image) and the AlbumCardItem grid + total.
  - New `ui/label/LabelReleasesView.slint`: label header card,
    then the same absolute-position album grid + load-more pattern
    the artist/musician views use. Reuses the Discover AlbumCard
    (artist-as-subtitle is correct here — a label's catalog is
    various artists).
  - AppShell routes ContentView.label and forwards open-album +
    media-action.

Rust:
  - `crate::label` controller — load_label (parallel-ish
    get_label_page + get_label_albums), load_more_albums,
    apply_label, append_albums, apply_image (label header), and
    artwork_jobs. extract_label_image handles the flexible
    /label/page image value (string or mega/extralarge/.../small
    object), same precedence as the Svelte.
  - artwork::ArtworkTarget gains LabelAlbum { index }.
  - main.rs: navigate_label + apply_entry route + on_label_clicked
    (sidebar) + LabelActions::on_load_more.
  - No new qbz-core methods — get_label_page / get_label_albums
    already existed.

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, AlbumState, ArtistState, NavState, SessionState, SettingsState, AlbumActions, ArtistActions, ArtworkActions, NowPlayingState, QueueState, SearchState, SearchActions, NetworkSidebarState, NetworkSidebarActions, MusicianState, MusicianActions } from "state.slint";
+export { HomeState, AlbumState, ArtistState, NavState, SessionState, SettingsState, AlbumActions, ArtistActions, ArtworkActions, NowPlayingState, QueueState, SearchState, SearchActions, NetworkSidebarState, NetworkSidebarActions, MusicianState, MusicianActions, LabelState, LabelActions } 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`.