Rebuild stack-folder with sticky tab rail and site previews.

L0–L7 folders stack on scroll with aligned max depth, labeled tabs that
stay consistent when L7 joins the rail, Cal embeds, preview screenshots,
Playwright tests, and updated README.

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

package.json

@@ -6,7 +6,10 @@
   "scripts": {
     "dev": "vite",
     "build": "vite build",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "capture-previews": "node scripts/capture-previews.mjs",
+    "test:folder": "node scripts/test-stack-folder.mjs",
+    "test:stack-scroll": "node scripts/test-stack-scroll.mjs"
   },
   "devDependencies": {
     "playwright": "^1.60.0",