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/assets/icons/folder-open.svg

@@ -0,0 +1,3 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#ffffff" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+  <path d="m6 14 1.5-2.9A2 2 0 0 1 9.24 10H20a2 2 0 0 1 1.94 2.5l-1.54 6a2 2 0 0 1-1.95 1.5H4a2 2 0 0 1-2-2V5a2 2 0 0 1 2-2h3.93a2 2 0 0 1 1.66.9l.82 1.2a2 2 0 0 0 1.66.9H18a2 2 0 0 1 2 2v2" />
+</svg>

crates/qbz-slint/ui/assets/icons/folder-plus.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#ffffff" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+  <path d="M12 10v6" />
+  <path d="M9 13h6" />
+  <path d="M20 20a2 2 0 0 0 2-2V8a2 2 0 0 0-2-2h-7.9a2 2 0 0 1-1.69-.9L9.6 3.9A2 2 0 0 0 7.93 3H4a2 2 0 0 0-2 2v13a2 2 0 0 0 2 2Z" />
+</svg>