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/state.slint

@@ -94,7 +94,7 @@ export global AlbumActions {
 }
 
 // Which content view the shell shows.
-export enum ContentView { home, album, artist, settings, search, musician }
+export enum ContentView { home, album, artist, settings, search, musician, label }
 
 // Settings screen state. Audio and Playback preferences are loaded from
 // the persistence stores by the Rust settings controller and pushed here;
@@ -433,6 +433,24 @@ export global MusicianActions {
     callback load-more();
 }
 
+// LabelReleasesView state — opened by the artist network sidebar's
+// Labels section. Header (name + image) from /label/page, the
+// album grid from /label/getAlbums. Populated by `crate::label`.
+export global LabelState {
+    in property <string> id;
+    in property <string> name;
+    in property <string> image-url;
+    in property <image> image;
+    in property <[AlbumCardItem]> albums: [];
+    in property <int> total: 0;
+    in property <bool> loading: false;
+    in property <bool> load-more-loading: false;
+}
+
+export global LabelActions {
+    callback load-more();
+}
+
 // Shell chrome state — collapsible left sidebar and right-side panels.
 // Globals are imported directly by the shell components, so toggles
 // need no callback plumbing.