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/stores/stylesheet.js

@@ -0,0 +1,38 @@
+import { defineStore } from 'pinia';
+import { ref } from 'vue';
+import cssParsingUtils from '../utils/css-parsing';
+
+export const useStylesheetStore = defineStore('stylesheet', () => {
+  const content = ref('');
+
+  const loadStylesheet = async () => {
+    const response = await fetch('/assets/css/stylesheet.css');
+    content.value = await response.text();
+  };
+
+  const updateProperty = (selector, property, value, unit) => {
+    content.value = cssParsingUtils.updateCssValue({
+      css: content.value,
+      selector,
+      property,
+      value,
+      unit
+    });
+  };
+
+  const extractValue = (selector, property) => {
+    return cssParsingUtils.extractCssValue(content.value, selector, property);
+  };
+
+  const extractBlock = (selector) => {
+    return cssParsingUtils.extractCssBlock(content.value, selector);
+  };
+
+  return {
+    content,
+    loadStylesheet,
+    updateProperty,
+    extractValue,
+    extractBlock
+  };
+});