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/composables/useDebounce.js

@@ -0,0 +1,15 @@
+/**
+ * Composable for debounced updates
+ * @param {number} delay - Debounce delay in milliseconds (default: 500ms)
+ * @returns {Function} debouncedUpdate function
+ */
+export function useDebounce(delay = 500) {
+  let timer = null;
+
+  const debouncedUpdate = (callback) => {
+    clearTimeout(timer);
+    timer = setTimeout(callback, delay);
+  };
+
+  return { debouncedUpdate };
+}