0.5.2: paste images, shared import helper, clipart persistence

- Clipboard paste creates an image layer: a document-level paste
  listener detects image items in the clipboard (Strg+V after a
  screenshot, e.g.) and imports them via the shared helper. Skipped
  when focus is in a real text input / textarea / contenteditable.
- Shared importImageFile() helper in fileLoader.ts replaces the
  duplicated image-import logic in AssetSidebar and CanvasStage's
  clipart-drop path. Optional position argument for drop-at-point.
- Clipart folder persistence hardened for reloads:
  - initialize() is now safely re-runnable (no-ops only once a handle
    is actually loaded, tolerates refresh failures so the folder name
    survives even if permission isn't auto-granted)
  - App.vue's onMounted kicks off initialize() so it runs regardless
    of whether the drawer component has mounted
  - DocumentSettings re-invokes initialize() each time the modal
    opens — covers app-start races and gives the user gesture needed
    for FS Access permission prompts
  - Opening the drawer refreshes thumbnails when handle is set but
    items are empty, so the user-gesture context restores permission

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

package.json

@@ -1,7 +1,7 @@
 {
   "name": "animatr",
   "private": true,
-  "version": "0.5.1",
+  "version": "0.5.2",
   "type": "module",
   "scripts": {
     "dev": "vite",