feat(web-admin): add image preview transform helpers for fit/zoom.

Pure functions for AttachmentUploadField image preview rotate/zoom math, with a Node assert script.

Co-authored-by: Cursor <cursoragent@cursor.com>

web-admin/scripts/test-image-preview-rotate.mjs

@@ -0,0 +1,34 @@
+import assert from 'node:assert/strict'
+import {
+  IMAGE_PREVIEW_ZOOM_MIN,
+  IMAGE_PREVIEW_ZOOM_MAX,
+  IMAGE_PREVIEW_ZOOM_STEP,
+  clampPreviewZoom,
+  computeFitBaseScale,
+  buildPreviewTransformCss
+} from '../src/utils/image-preview-transform.js'
+
+assert.equal(IMAGE_PREVIEW_ZOOM_MIN, 0.5)
+assert.equal(IMAGE_PREVIEW_ZOOM_MAX, 5)
+assert.equal(IMAGE_PREVIEW_ZOOM_STEP, 0.15)
+
+assert.equal(clampPreviewZoom(0.1), 0.5)
+assert.equal(clampPreviewZoom(9), 5)
+assert.equal(clampPreviewZoom(1.234), 1.23)
+
+// 未旋转：640x400 装入 320x200 → scale = 0.5
+assert.ok(
+  Math.abs(computeFitBaseScale(640, 400, 0, 320, 200) - 0.5) < 1e-9
+)
+
+// 旋转 90°：外接 400x640 装入 320x200 → scale = min(320/400, 200/640) = 0.3125
+assert.ok(
+  Math.abs(computeFitBaseScale(640, 400, 90, 320, 200) - 0.3125) < 1e-9
+)
+
+assert.equal(
+  buildPreviewTransformCss(10, -20, 90, 0.5),
+  'translate(10px, -20px) rotate(90deg) scale(0.5)'
+)
+
+console.log('image-preview-transform ok')