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/GridSection.slint

@@ -0,0 +1,50 @@
+// A Discover section that lays slim cards in a fixed 4-column grid
+// (Popular albums, Recently Played). Non-paginated for now.
+//
+// GridLayout cannot host a `for`, so cards are absolutely positioned
+// inside a Rectangle whose height is derived from the row count.
+
+import { Theme } from "../foundation/semantic-colors.slint";
+import { Typography } from "../foundation/typography.slint";
+import { SlimItem } from "../state.slint";
+import { SlimCard } from "SlimCard.slint";
+
+export component GridSection inherits VerticalLayout {
+    in property <string> title;
+    in property <[SlimItem]> items;
+    callback item-clicked(string);
+
+    property <int> columns: 4;
+    property <length> gap: 8px;
+    property <length> row-height: 60px;
+    property <int> rows: Math.ceil(root.items.length / root.columns);
+
+    spacing: 12px;
+
+    Text {
+        text: root.title;
+        color: Theme.text-primary;
+        font-size: 18px;
+        font-weight: Typography.semibold;
+    }
+
+    grid := Rectangle {
+        property <length> cell-width:
+            (self.width - (root.columns - 1) * root.gap) / root.columns;
+
+        height: root.rows > 0
+            ? root.rows * root.row-height + (root.rows - 1) * root.gap
+            : 0px;
+
+        for item[i] in root.items: SlimCard {
+            width: grid.cell-width;
+            height: root.row-height;
+            x: Math.mod(i, root.columns) * (grid.cell-width + root.gap);
+            y: Math.floor(i / root.columns) * (root.row-height + root.gap);
+            item: item;
+            clicked(id) => {
+                root.item-clicked(id);
+            }
+        }
+    }
+}