feat(slint): playlist folders in the sidebar

Local, Tauri-shared playlist folders (library.db) now organize the
sidebar:

  - New folders.rs controller (create / rename / delete / move via
    qbz-library); sidebar.rs reworked to a flattened entries model
    (folder headers + grouped playlists + root playlists) with a
    session expand cache, rebuilt without a refetch on expand/move.
  - Sidebar renders folders (expandable, with playlist counts) and
    playlists (indented inside folders); a hover "..." opens a context
    popup: Move to folder (root + each folder) for playlists, Delete
    folder for folders.
  - Header gains a New-folder button + a CreateFolderModal; folders
    reload after create/delete; moves are optimistic + persisted.

Adds folder-open / folder-plus icons.

cc

crates/qbz-slint/ui/shell/AppShell.slint

@@ -13,6 +13,7 @@ import { HeaderMenuOverlay } from "HeaderMenuOverlay.slint";
 import { PlaylistPickerModal } from "../primitives/PlaylistPickerModal.slint";
 import { CreatePlaylistModal } from "../primitives/CreatePlaylistModal.slint";
 import { EditPlaylistModal } from "../primitives/EditPlaylistModal.slint";
+import { CreateFolderModal } from "../primitives/CreateFolderModal.slint";
 import { PlayerBar } from "PlayerBar.slint";
 import { QueueSidebar } from "QueueSidebar.slint";
 import { LyricsSidebar } from "LyricsSidebar.slint";
@@ -233,4 +234,7 @@ export component AppShell inherits Rectangle {
 
     // Global "Edit playlist" modal (rename / delete).
     EditPlaylistModal { }
+
+    // Global "Create folder" modal.
+    CreateFolderModal { }
 }