refboard: complete image geometry rework — consumer adoption + tests

- Fix grouping.ts ungroup: transform display position (with flip
  compensation) through group transform, then back-calculate canonical
  data.x/y. Fixes visual jump for flipped images in rotated groups.
- Fix clipboard.ts + export.ts resolution estimation: account for
  scale when computing effective rendered width for texture ratio.
- Add imageTransforms.test.ts: 32 unit tests covering source rect,
  visible local rect, display transform, display geometry, editor
  geometry anchor preservation, crop rect round-trips, coordinate
  conversions, negative scale normalization, and world bounds invariants.
- Add vitest as dev dependency with "test" script.
- Include canonical geometry layer (imageTransforms.ts) and crop
  session rewrite (CropOverlay.ts) from prior work.
- Add image geometry rework plan document.

frontend/src/canvas/clipboard.ts

@@ -91,8 +91,8 @@ export async function writeCanvasToClipboard(
     for (const item of items) {
       const tex = (item.displayObject as any)?.texture;
       const displayW = item.type === 'image'
-        ? getImageVisibleLocalRect(item.data as ImageObject).w
-        : item.data.w;
+        ? getImageVisibleLocalRect(item.data as ImageObject).w * Math.abs(item.data.sx)
+        : item.data.w * Math.abs(item.data.sx);
       if (tex && tex.width > 1 && displayW > 1) {
         maxRatio = Math.max(maxRatio, tex.width / displayW);
       }