feat: iframe-based PagedJS preview with reactive CSS editor

- Isolate PagedJS in iframe to avoid DOM/CSS conflicts
- Add EditorPanel for global CSS controls
- Add StylesheetViewer with highlight.js syntax highlighting
- Add ElementPopup for element-specific CSS editing
- CSS modifications update preview reactively
- Support px/rem/em units

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -9,6 +9,7 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "highlight.js": "^11.11.1",
     "pagedjs": "^0.4.3",
     "vue": "^3.5.24"
   },