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>

src/main.js

@@ -1,5 +1,8 @@
-import { createApp } from 'vue'
-import './style.css'
-import App from './App.vue'
+import { createApp } from 'vue';
+import { createPinia } from 'pinia';
+import './style.css';
+import App from './App.vue';
 
-createApp(App).mount('#app')
+const app = createApp(App);
+app.use(createPinia());
+app.mount('#app');