feat(qbz-slint): artist detail page

Clicking an artist (from the album page's artist line) opens the
artist detail view: a circular portrait, name, biography, action
buttons, a Popular Tracks list, and a Discography carousel.

- artist.rs: fetches the artist page via QbzCore::get_artist_page and
  maps name, biography (HTML-stripped), portrait URL, top tracks and
  releases; the portrait loads through the shared image cache.
- ArtistPageView.slint reuses AlbumCard for the discography (through
  the Carousel) and AlbumTrackItem for the track list.
- NavState becomes a three-way ContentView (home / album / artist) and
  AppShell routes the content region accordingly. The album page's
  artist line and the artist page's release cards are navigable.

cc

crates/qbz-slint/ui/album/AlbumPageView.slint

@@ -9,7 +9,7 @@ import { Theme } from "../foundation/semantic-colors.slint";
 import { Typography } from "../foundation/typography.slint";
 import { Radius } from "../foundation/radius.slint";
 import { Spacing } from "../foundation/spacing.slint";
-import { AlbumState, AlbumTrackItem, NavState } from "../state.slint";
+import { AlbumState, AlbumTrackItem, NavState, ContentView } from "../state.slint";
 import { QbzIcon } from "../primitives/QbzIcon.slint";
 import { ScrollView } from "std-widgets.slint";
 
@@ -132,6 +132,8 @@ component TrackRow inherits Rectangle {
 }
 
 export component AlbumPageView inherits Rectangle {
+    callback open-artist(string);
+
     background: Theme.surface-main;
 
     ScrollView {
@@ -150,7 +152,7 @@ export component AlbumPageView inherits Rectangle {
                 height: 28px;
                 mouse-cursor: pointer;
                 clicked => {
-                    NavState.showing-album = false;
+                    NavState.view = ContentView.home;
                 }
                 back-row := HorizontalLayout {
                     spacing: 6px;
@@ -210,12 +212,24 @@ export component AlbumPageView inherits Rectangle {
                         overflow: elide;
                     }
                     Rectangle { height: 4px; }
-                    Text {
-                        text: AlbumState.artist;
-                        color: Theme.accent;
-                        font-size: Typography.heading;
-                        font-weight: Typography.medium;
-                        overflow: elide;
+                    HorizontalLayout {
+                        alignment: start;
+                        artist-area := TouchArea {
+                            width: artist-text.preferred-width;
+                            height: artist-text.preferred-height;
+                            mouse-cursor: pointer;
+                            clicked => {
+                                root.open-artist(AlbumState.artist-id);
+                            }
+                            artist-text := Text {
+                                text: AlbumState.artist;
+                                color: artist-area.has-hover
+                                    ? Theme.accent-hover
+                                    : Theme.accent;
+                                font-size: Typography.heading;
+                                font-weight: Typography.medium;
+                            }
+                        }
                     }
                     Rectangle { height: 10px; }
                     Text {