refactor: extract debounce logic into shared composable

- Create useDebounce composable to avoid code duplication
- Apply debounce to TextSettings margin/padding inputs
- Harmonize debounce delay to 500ms across all components
- Fix input lag when typing values like "30mm"

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/components/PagePopup.vue

@@ -285,6 +285,7 @@
 import { ref, computed, watch, onMounted } from 'vue';
 import { useStylesheetStore } from '../stores/stylesheet';
 import { usePopupPosition } from '../composables/usePopupPosition';
+import { useDebounce } from '../composables/useDebounce';
 import NumberInput from './ui/NumberInput.vue';
 import Coloris from '@melloware/coloris';
 import '@melloware/coloris/dist/coloris.css';
@@ -312,7 +313,7 @@ const inheritanceLocked = ref(true);
 const backgroundColorInput = ref(null);
 
 let isUpdatingFromStore = false;
-let updateTimer = null;
+const { debouncedUpdate } = useDebounce(500);
 
 const margins = ref({
   top: { value: 0, unit: 'mm' },
@@ -328,11 +329,6 @@ const background = ref({
 
 const pattern = ref('');
 
-const debouncedUpdate = (callback) => {
-  clearTimeout(updateTimer);
-  updateTimer = setTimeout(callback, 1000);
-};
-
 const immediateUpdate = (callback) => {
   callback();
 };