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/package.json

@@ -6,7 +6,8 @@
   "scripts": {
     "dev": "vite",
     "build": "vite build",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "test": "vitest run"
   },
   "dependencies": {
     "@blocknote/core": "^0.47.1",
@@ -31,6 +32,7 @@
     "@types/turndown": "^5.0.6",
     "@vitejs/plugin-react": "^4.3.4",
     "typescript": "^5.6.3",
-    "vite": "^6.0.3"
+    "vite": "^6.0.3",
+    "vitest": "^4.1.0"
   }
 }