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/i18n/messages.ts

@@ -395,6 +395,8 @@ export const en = {
     width: 'Width',
     height: 'Height',
     background: 'Background',
+    backgroundTransparent: 'Transparent',
+    backgroundTransparentTooltip: 'Transparent background (APNG exports keep alpha)',
     duration: 'Duration (s)',
     fps: 'FPS',
     timelineHint: 'Timeline length sets the ruler scale. Playback stops at the last keyframe.',
@@ -417,11 +419,21 @@ export const en = {
     webmBadge: 'Browser · Realtime',
     webmDesc: 'No server — recorded in real time',
     webmNotSupported: 'Browser does not support MediaRecorder',
+    gifBadge: 'Browser · Frame-by-frame',
+    gifDesc: '256-colour palette, broadest compatibility',
+    apngBadge: 'Browser · Lossless',
+    apngDesc: 'Full RGBA with transparency — .png file',
+    apngPreviewTitle: 'APNG preview page',
+    apngPreviewBadge: 'HTML · Checkerboard',
+    apngPreviewDesc: 'Standalone HTML with a checkerboard backdrop so you can verify transparency',
     cancel: 'Cancel',
     failed: 'Export failed',
     back: 'Back',
     labelWebm: 'Rendering WebM (browser, realtime)…',
     labelMp4: 'Rendering MP4 (server)…',
+    labelGif: 'Rendering GIF (browser, frame-by-frame)…',
+    labelApng: 'Rendering APNG (browser, frame-by-frame)…',
+    labelApngPreview: 'Rendering APNG + preview page…',
     skipHidden: 'Skip hidden tracks',
     skipHiddenDesc: 'Drop layers toggled off in the editor from every format below.',
     skipHiddenNone: 'No editor-hidden tracks in the scene.',
@@ -1174,6 +1186,8 @@ export const de: DeepPartial<Messages> = {
     width: 'Breite',
     height: 'Höhe',
     background: 'Hintergrund',
+    backgroundTransparent: 'Transparent',
+    backgroundTransparentTooltip: 'Transparenter Hintergrund (beim APNG-Export bleibt Alpha erhalten)',
     duration: 'Dauer (s)',
     fps: 'FPS',
     timelineHint: 'Die Timeline-Länge bestimmt die Skala im Lineal. Die Wiedergabe stoppt am Ende.',
@@ -1195,11 +1209,21 @@ export const de: DeepPartial<Messages> = {
     webmBadge: 'Browser · Echtzeit',
     webmDesc: 'Ohne Server: direkt im Browser aufgenommen.',
     webmNotSupported: 'Dieser Browser unterstützt MediaRecorder nicht.',
+    gifBadge: 'Browser · Frame-für-Frame',
+    gifDesc: '256-Farben-Palette, maximale Kompatibilität.',
+    apngBadge: 'Browser · Verlustfrei',
+    apngDesc: 'Volles RGBA mit Transparenz — .png-Datei.',
+    apngPreviewTitle: 'APNG-Vorschauseite',
+    apngPreviewBadge: 'HTML · Schachbrett',
+    apngPreviewDesc: 'Eigenständige HTML-Datei mit Schachbrett-Hintergrund zum Transparenz-Check.',
     cancel: 'Abbrechen',
     failed: 'Export fehlgeschlagen',
     back: 'Zurück',
     labelWebm: 'WebM wird gerendert (Browser, Echtzeit)…',
     labelMp4: 'MP4 wird gerendert (Server)…',
+    labelGif: 'GIF wird gerendert (Browser, frame-für-frame)…',
+    labelApng: 'APNG wird gerendert (Browser, frame-für-frame)…',
+    labelApngPreview: 'APNG + Vorschau werden gerendert…',
     skipHidden: 'Versteckte Tracks überspringen',
     skipHiddenDesc: 'Lässt editor-versteckte Ebenen in allen Exportformaten weg.',
     skipHiddenNone: 'In dieser Szene gibt es keine editor-versteckten Tracks.',