0.4.0: clipart drawer, animation paths, bezier keyframes

- Clipart library: new left-edge drawer (5px idle, expands on hover,
  400px when open). User picks a local folder via the File System Access
  API in settings; directory handle is persisted in IndexedDB so the
  library survives reloads. Dragging a thumbnail onto the stage creates
  an image asset + layer at the drop point.
- Animation paths on the stage: selecting a keyframe draws the tween
  path from the previous keyframe to the selected one as a solid
  polyline with per-frame draggable dots. All other segments on the
  same position-track render as dashed ghost lines so the full curve
  progression is visible.
- Path dots grow on hover (2.5 → 6 px), show a grab cursor, and have a
  wider hit area so they're easier to grab.
- Bezier-curved position interpolation: keyframes gain optional
  ctrlX/ctrlY fields. When set, x/y tween along a quadratic bezier
  through the control point instead of a straight line. Dragging a
  path dot solves for the control so the curve passes through the
  drag point at its frame progress. Mirrored across all four render
  pipelines.
- Tests: new suites for polygon actions, freeform drawing lifecycle,
  polygon + stroke rendering (15 new tests). Fixed the mask-render
  tests that relied on a default-transparent canvas background.

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

src/i18n/messages.ts

@@ -221,6 +221,7 @@ export const en = {
     tab: {
       document: 'Document',
       timeline: 'Timeline',
+      clipart: 'Clipart',
       theme: 'UI Theme',
     },
     canvas: 'Canvas',
@@ -271,6 +272,23 @@ export const en = {
     failedLabel: '{n} failed',
   },
 
+  clipart: {
+    title: 'Clipart library',
+    open: 'Open clipart library',
+    close: 'Close clipart library',
+    refresh: 'Reload folder',
+    empty: 'Choose a folder in settings to populate the library.',
+    noFiles: 'No image files in this folder.',
+    loading: 'Loading…',
+    unsupported: 'Your browser does not support folder access. Try Chrome or Edge.',
+    chooseFolder: 'Choose folder…',
+    clearFolder: 'Clear',
+    folderLabel: 'Folder',
+    noFolder: 'No folder selected',
+    settingsHint:
+      'Point to a local folder with your SVG/PNG/JPG clipart. The app remembers the folder; permissions may be re-asked after browser restart.',
+  },
+
   dialog: {
     confirm: 'Confirm',
     input: 'Input',