feat(slint): add slim-card grid for Popular albums

Add a compact "slim" card variant and a 4-column grid section that
renders it, used for the Popular albums row on Discover Home.

- SlimCard: 60px row with optional rank, 44px thumbnail, title/subtitle
- GridSection: 4-column grid; cards absolutely positioned since Slint
  GridLayout cannot host a `for`
- state.slint: SlimItem struct and HomeState.popular model
- home.rs: load_home now returns HomeData { sections, popular };
  map_slim maps the discover most-streamed container
- artwork pipeline: ArtworkTarget enum routes decoded covers to either
  a section card or a popular card

cc

crates/qbz-slint/ui/discover/HomeView.slint

@@ -10,6 +10,7 @@ import { Theme } from "../foundation/semantic-colors.slint";
 import { Typography } from "../foundation/typography.slint";
 import { HomeState } from "../state.slint";
 import { Carousel } from "Carousel.slint";
+import { GridSection } from "GridSection.slint";
 import { QbzIcon } from "../primitives/QbzIcon.slint";
 import { ScrollView } from "std-widgets.slint";
 
@@ -174,6 +175,14 @@ export component HomeView inherits Rectangle {
                         root.album-clicked(id);
                     }
                 }
+
+                if HomeState.popular.length > 0: GridSection {
+                    title: @tr("Popular albums");
+                    items: HomeState.popular;
+                    item-clicked(id) => {
+                        root.album-clicked(id);
+                    }
+                }
             }
         }
     }