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>

src/version.ts

@@ -1,2 +1,2 @@
-export const APP_VERSION = '0.15.0'
+export const APP_VERSION = '0.16.0'
 export const APP_NAME = 'RUWIO'