refactor: implement Pinia store for stylesheet management

- Add Pinia state management
- Create centralized stylesheet store with utility methods
- Extract CSS parsing utilities to src/utils/css-parsing.js
- Refactor ElementPopup to manage state independently via store
- Simplify App.vue by removing prop drilling
- Fix iframe rendering with srcdoc instead of document.write
- Improve API: updateProperty uses object parameter for clarity

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

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

package.json

@@ -11,6 +11,7 @@
   "dependencies": {
     "highlight.js": "^11.11.1",
     "pagedjs": "^0.4.3",
+    "pinia": "^3.0.4",
     "vue": "^3.5.24"
   },
   "devDependencies": {