refactor: move shared utilities to lib/shared/ sub-packages

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/shared/utils/debounce.ts

@@ -0,0 +1,12 @@
+/**
+ * Returns a debounced version of fn that delays invocation until after
+ * `delay` ms have elapsed since the last call.
+ */
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export function debounce<T extends (...args: any[]) => void>(fn: T, delay: number): T {
+	let timer: ReturnType<typeof setTimeout>;
+	return ((...args: Parameters<T>) => {
+		clearTimeout(timer);
+		timer = setTimeout(() => fn(...args), delay);
+	}) as T;
+}