0.2.0: polygon tool, mask editor preview, collapsible properties

- Polygon asset type with per-vertex animation: new sidebar button inserts a
  hexagon; vertex handles in the editor write per-keyframe `points` arrays
  that interpolate between keyframes (requires matching point counts).
- Canvas mask preview: toggle in the zoom toolbar switches the stage
  between the editable ghost view and a rendered view that applies the
  actual masking via the shared canvas 2D path.
- Properties panel is grouped into collapsible sections (Transform, Text,
  Appearance, Shadow, Mask) with per-section open/closed state persisted
  to localStorage.
- Sidebar shape buttons are icon-only to reclaim width for the Polygon
  entry.

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

src/i18n/messages.ts

@@ -64,6 +64,7 @@ export const en = {
       rect: 'Rect',
       circle: 'Circle',
       text: 'Text',
+      polygon: 'Polygon',
     },
     newTextDialog: {
       title: 'New text object',
@@ -95,7 +96,14 @@ export const en = {
     removeKeyframe: 'Remove keyframe here',
     deleteLayer: 'Delete layer',
     noLayer: 'No layer selected.',
+    fill: 'Fill',
+    opacity: 'Opacity',
     edgeBlur: 'Edge blur',
+    sections: {
+      transform: 'Transform',
+      text: 'Text',
+      appearance: 'Appearance',
+    },
     shadow: 'Shadow',
     shadowBlur: 'Shadow blur',
     shadowOpacity: 'Shadow opacity',