0.16.0: animated GIF / APNG export + transparent canvas

- New frame-by-frame export pipeline in src/lib/frameExport.ts: loads
  scene images, walks rangeIn..rangeOut at scene.fps, drawSceneFrame
  to an offscreen canvas, awaits a per-frame callback, yields every
  4 frames to keep progress responsive, honours AbortSignal.
- Animated GIF via gifenc (per-frame quantize/palette, ~5 KB lib).
- Animated PNG via UPNG.js + pako (lossless RGBA, alpha-aware, ~30 KB).
- drawSceneFrame now clearRects first and skips the background fill
  when scene.canvas.background === 'transparent' — prerequisite for
  the alpha to reach the APNG output.
- DocumentDrawer: new checkerboard toggle next to the colour picker
  flips the stage between its last-picked colour and transparent;
  disabled swatch + 'Transparent' hex label make the state obvious.
- ExportDialog: three new option rows — Animated GIF, Animated PNG,
  and an APNG preview page that bundles the rendered APNG into a
  standalone HTML with a checkerboard backdrop + dark/light toggles
  for verifying transparency at a glance.
- Tests: renderFrames frame count + abort, GIF header magic, APNG
  acTL presence, transparent round-trip (decoded alpha == 0 on
  un-painted pixels), preview HTML structure, and the two UI tests
  covering ExportDialog's new rows and the progress-view transition.

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

package.json

@@ -1,7 +1,7 @@
 {
   "name": "ruwio",
   "private": true,
-  "version": "0.15.0",
+  "version": "0.16.0",
   "type": "module",
   "scripts": {
     "dev": "vite",
@@ -12,8 +12,10 @@
   },
   "dependencies": {
     "@phosphor-icons/vue": "^2.2.1",
+    "gifenc": "^1.0.3",
     "konva": "^9.3.16",
     "pinia": "^2.2.6",
+    "upng-js": "^2.1.0",
     "vue": "^3.5.13",
     "vue-konva": "^3.1.2"
   },