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

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

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

@@ -0,0 +1,19 @@
+export type SortDir = 'ASC' | 'DESC';
+
+/**
+ * Returns a new array of documents sorted by documentDate.
+ * Documents without a date are always placed last, regardless of direction.
+ */
+export function sortDocumentsByDate<T extends { documentDate?: string | null }>(
+	docs: T[],
+	dir: SortDir
+): T[] {
+	return [...docs].sort((a, b) => {
+		const da = a.documentDate ?? '';
+		const db = b.documentDate ?? '';
+		if (!da && !db) return 0;
+		if (!da) return 1;
+		if (!db) return -1;
+		return dir === 'DESC' ? db.localeCompare(da) : da.localeCompare(db);
+	});
+}