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

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

frontend/src/lib/shared/discussion/commentDeepLink.ts

@@ -0,0 +1,8 @@
+export function buildCommentHref(
+	documentId: string,
+	commentId: string,
+	annotationId: string | null
+): string {
+	const base = `/documents/${documentId}?commentId=${commentId}`;
+	return annotationId ? `${base}&annotationId=${annotationId}` : base;
+}