feat: improve code editor formatting behavior

- Disable auto-formatting while in editing mode
- Format CSS only when exiting editing mode
- Auto-disable editing mode when switching tabs

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

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

src/stores/stylesheet.js

@@ -6,6 +6,7 @@ import parserPostcss from 'prettier/plugins/postcss';
 
 export const useStylesheetStore = defineStore('stylesheet', () => {
   const content = ref('');
+  const isEditing = ref(false);
   let formatTimer = null;
   let isFormatting = false;
 
@@ -30,9 +31,9 @@ export const useStylesheetStore = defineStore('stylesheet', () => {
     }
   };
 
-  // Watch content and format after 500ms of inactivity
+  // Watch content and format after 500ms of inactivity (only when not editing)
   watch(content, () => {
-    if (isFormatting) return;
+    if (isFormatting || isEditing.value) return;
 
     clearTimeout(formatTimer);
     formatTimer = setTimeout(() => {
@@ -65,6 +66,7 @@ export const useStylesheetStore = defineStore('stylesheet', () => {
 
   return {
     content,
+    isEditing,
     loadStylesheet,
     updateProperty,
     extractValue,