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/editor/PageSettings.vue

@@ -288,16 +288,17 @@
 <script setup>
 import { ref, computed, watch, onMounted, inject } from 'vue';
 import { useStylesheetStore } from '../../stores/stylesheet';
+import { useDebounce } from '../../composables/useDebounce';
 import Coloris from '@melloware/coloris';
 import NumberInput from '../ui/NumberInput.vue';
 import '@melloware/coloris/dist/coloris.css';
 
 const stylesheetStore = useStylesheetStore();
+const { debouncedUpdate } = useDebounce(500);
 const backgroundColorInput = ref(null);
 const activeTab = inject('activeTab', ref('document'));
 
 let isUpdatingFromStore = false;
-let updateTimer = null;
 
 const pageFormat = ref('A4');
 
@@ -328,11 +329,6 @@ const pattern = ref('');
 const pageNumbers = ref(false);
 const runningTitle = ref(false);
 
-const debouncedUpdate = (callback) => {
-  clearTimeout(updateTimer);
-  updateTimer = setTimeout(callback, 1000);
-};
-
 const immediateUpdate = (callback) => {
   callback();
 };