feat(slint): sidebar Playlists toolbar + folders + micro-collage refinements

PLAYLISTS header: smaller title, recursive search (hides title), create
modal at 1:1 with Tauri (name/description/folder/public), sort menu, and a
collapse chevron. Folders: accent-tinted icon, open/close chevron as the
primary affordance (no more purposeless "..."), count shown on hover only.
Playlists render a 2x2 micro-collage of track covers (opt-out) instead of
the glyph, sourced from the existing playlist images (no extra fetch).

cc

crates/qbz-slint/ui/primitives/SidebarPlaylistCollage.slint

@@ -0,0 +1,69 @@
+// Sidebar playlist micro-collage. A tiny (20px) thumbnail of a
+// playlist's track covers shown in place of the generic list-music
+// glyph, mirroring Tauri's PlaylistCoverCollage:
+//   - 0 covers      -> parent renders the fallback glyph (nothing here)
+//   - 1-3 covers     -> a single cover (the first tile), full bleed
+//   - 4+ covers      -> a 2x2 grid, 1px gap, 3px radius
+//
+// The covers + decoded images live on the SidebarEntry (cover-count,
+// cover1..cover4). This component only lays them out.
+
+import { Theme } from "../foundation/semantic-colors.slint";
+import { SidebarEntry } from "../state.slint";
+
+export component SidebarPlaylistCollage inherits Rectangle {
+    in property <SidebarEntry> entry;
+    in property <length> size: 20px;
+
+    width: root.size;
+    height: root.size;
+    border-radius: 3px;
+    clip: true;
+    background: Theme.surface-elevated;
+
+    property <int> n: root.entry.cover-count;
+    property <length> gap: 1px;
+
+    // 1-3 covers -> single full-bleed cover (Tauri: tiles.length < 4).
+    if root.n >= 1 && root.n < 4: Image {
+        width: 100%;
+        height: 100%;
+        source: root.entry.cover1;
+        image-fit: cover;
+    }
+
+    // 4+ covers -> 2x2 grid (Tauri: tiles.slice(0, 4)).
+    if root.n >= 4: VerticalLayout {
+        spacing: root.gap;
+        HorizontalLayout {
+            spacing: root.gap;
+            Image {
+                source: root.entry.cover1;
+                image-fit: cover;
+                width: (root.size - root.gap) / 2;
+                height: (root.size - root.gap) / 2;
+            }
+            Image {
+                source: root.entry.cover2;
+                image-fit: cover;
+                width: (root.size - root.gap) / 2;
+                height: (root.size - root.gap) / 2;
+            }
+        }
+        HorizontalLayout {
+            spacing: root.gap;
+            Image {
+                source: root.entry.cover3;
+                image-fit: cover;
+                width: (root.size - root.gap) / 2;
+                height: (root.size - root.gap) / 2;
+            }
+            Image {
+                source: root.entry.cover4;
+                image-fit: cover;
+                width: (root.size - root.gap) / 2;
+                height: (root.size - root.gap) / 2;
+            }
+        }
+    }
+}