feat: add automatic CSS formatting with Prettier

Integrates Prettier into the stylesheet store for automatic CSS formatting:
- Installs prettier v3.7.4 with postcss plugin
- Implements formatContent() function using Prettier API
- Auto-formats CSS after 500ms of inactivity (debounced)
- Prevents infinite loops with isFormatting flag
- Ensures consistent indentation and line breaks
- Cleans up extra blank lines and formatting issues

This ensures the CSS in the Code tab stays clean and properly formatted
after reactive edits from TextSettings and PageSettings panels.

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

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

src/stores/stylesheet.js

@@ -1,9 +1,44 @@
 import { defineStore } from 'pinia';
-import { ref } from 'vue';
+import { ref, watch } from 'vue';
 import cssParsingUtils from '../utils/css-parsing';
+import prettier from 'prettier/standalone';
+import parserPostcss from 'prettier/plugins/postcss';
 
 export const useStylesheetStore = defineStore('stylesheet', () => {
   const content = ref('');
+  let formatTimer = null;
+  let isFormatting = false;
+
+  // Format CSS with Prettier
+  const formatContent = async () => {
+    if (isFormatting || !content.value) return;
+
+    try {
+      isFormatting = true;
+      const formatted = await prettier.format(content.value, {
+        parser: 'css',
+        plugins: [parserPostcss],
+        printWidth: 80,
+        tabWidth: 2,
+        useTabs: false,
+      });
+      content.value = formatted;
+    } catch (error) {
+      console.error('CSS formatting error:', error);
+    } finally {
+      isFormatting = false;
+    }
+  };
+
+  // Watch content and format after 500ms of inactivity
+  watch(content, () => {
+    if (isFormatting) return;
+
+    clearTimeout(formatTimer);
+    formatTimer = setTimeout(() => {
+      formatContent();
+    }, 500);
+  });
 
   const loadStylesheet = async () => {
     const response = await fetch('/assets/css/stylesheet.css');
@@ -33,6 +68,7 @@ export const useStylesheetStore = defineStore('stylesheet', () => {
     loadStylesheet,
     updateProperty,
     extractValue,
-    extractBlock
+    extractBlock,
+    formatContent
   };
 });