1.0.1: target size for GIF / APNG / APNG preview

Frame-based exports (GIF, APNG, APNG-preview) accept an optional
target output size. ExportDialog exposes it as a single width
input above the three image-export rows; height is derived live
from the scene's aspect ratio so the user can't squash the output.
The preview HTML mirrors the target dims so a downsized export
previews at its real pixel size.

renderFrames takes targetWidth / targetHeight, sizes its canvas
accordingly and scales the ctx once; drawSceneFrame's own
save/restore preserves the transform across frames, including the
top-level-mask offscreen compositing path.

Target width on dialog open is inherited from the current
scene.canvas — watched via modelValue so resizing the stage and
reopening the dialog picks up the new size (instead of the stale
value from whenever ExportDialog first instantiated).

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

src/i18n/messages.ts

@@ -451,6 +451,9 @@ export const en = {
     skipHidden: 'Skip hidden tracks',
     skipHiddenDesc: 'Drop layers toggled off in the editor from every format below.',
     skipHiddenNone: 'No editor-hidden tracks in the scene.',
+    targetSize: 'Target size',
+    targetSizeReset: 'Reset to scene canvas size',
+    targetSizeResetLabel: 'Reset',
   },
 
   tests: {