0.14.0: multi-folder media drawer

- Clipart store holds an array of root folders instead of a single
  handle, plus an activeHandleIndex for the in-folder view. Getters:
  activeHandle, inOverview, hasAnyFolder, canGoBack, folderName.
- Actions: addFolder (appends a root), enterFolder, exitFolder,
  removeFolder. Single-folder installs auto-drill into the sole root
  and behave exactly like the pre-refactor drawer; adding a 2nd+
  folder pops back to the root overview.
- IDB persistence migrated from the 'clipartDir' single-handle key to
  a 'clipartDirs' array. Legacy records are migrated on first load.
  saveClipartFolders unwraps Pinia proxies (toRaw per element) so
  IDB's structured clone accepts the payload.
- ClipartItem union gained a 'folder' kind so root tiles render as
  first-class media items. listClipart emits nested directories too,
  but the drawer keeps drill-down one level deep for now.
- ClipartDrawer header: three equally-sized .header-btn chips —
  back (only when canGoBack), refresh (when viewing a folder), and a
  new `+` (when any folder is attached). No more big 'Choose folder'
  button — empty state is a centred 96px dashed + with a wrapped hint.
- Folder tiles carry a hover `×` to remove a root from the library.
- DocumentSettings media tab lists all configured folders with a per-
  folder remove action; primary button is now 'Add folder'.
- Pinia store now calls acceptHMRUpdate so Vite HMR actually swaps
  the store definition between reloads instead of leaving the first-
  load state shape behind.
- Tests: store actions (addFolder / enterFolder / exitFolder / remove
  / IDB round-trip), drawer UI (centred + empty state, overview with
  folder tiles + click-to-enter, back visibility, single-root hides
  back, header + button triggers addFolder). Tests isolated with
  try/finally so a failing store assertion can't leak error state
  into subsequent drawer tests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/CanvasStage.vue

@@ -2809,7 +2809,7 @@ function onContainerDrop(e: DragEvent) {
     e.preventDefault()
     const idx = Number.parseInt(clipartIdx, 10)
     const item = Number.isFinite(idx) ? clipart.items[idx] : null
-    if (item) {
+    if (item && item.kind !== 'folder') {
       void importClipartAt(item.file, worldX, worldY)
     }
     return