feat(slint): playlist collage + custom artwork + edit description

Wires qbz-library into the Slint app (new library_db module) so the
local, Tauri-shared organization data is read/written from the same
per-user library.db (<data_dir>/qbz/users/<id>/library.db). The DB is
opened per-operation on a blocking thread (rusqlite Connection is
!Send).

  - Header collage: the playlist cover is now a 4-up collage of the
    first tracks' covers (Tauri parity); falls back to the server
    image, then the first track, then a placeholder.
  - Custom artwork: owner-only buttons on the cover set/clear a custom
    image. Set copies the picked file into <cache_dir>/qbz/artwork and
    stores the path in playlist_settings.custom_artwork_path (shared
    with Tauri); it's loaded directly from disk and overrides the
    collage.
  - Edit description: the edit modal gains a description field
    (prefilled), saved through update_playlist alongside the name.

cc

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

@@ -6,7 +6,7 @@ import { Typography } from "../foundation/typography.slint";
 import { Radius } from "../foundation/radius.slint";
 import { EditPlaylistState, EditPlaylistActions } from "../state.slint";
 import { QbzIcon } from "QbzIcon.slint";
-import { LineEdit } from "std-widgets.slint";
+import { LineEdit, TextEdit } from "std-widgets.slint";
 
 export component EditPlaylistModal inherits Rectangle {
     visible: EditPlaylistState.open;
@@ -71,6 +71,17 @@ export component EditPlaylistModal inherits Rectangle {
                     }
                 }
 
+                Text {
+                    text: @tr("Description");
+                    color: Theme.text-muted;
+                    font-size: Typography.legal;
+                }
+                desc-input := TextEdit {
+                    height: 90px;
+                    wrap: word-wrap;
+                    text <=> EditPlaylistState.description;
+                }
+
                 HorizontalLayout {
                     spacing: 10px;
                     // Delete (destructive, left).