feat(utils): add buildCommentHref helper for comment deep-links

Single source of truth for constructing /documents/:id?commentId=…
(&annotationId=…) URLs. Used by the notification bell, the chronik
"Für dich" sidebar, and the chronik main feed so the three surfaces
can no longer diverge.

Refs #300.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/lib/utils/commentDeepLink.spec.ts

@@ -0,0 +1,14 @@
+import { describe, it, expect } from 'vitest';
+import { buildCommentHref } from './commentDeepLink';
+
+describe('buildCommentHref', () => {
+	it('includes both commentId and annotationId when annotationId is present', () => {
+		const href = buildCommentHref('doc-1', 'comment-2', 'annot-3');
+		expect(href).toBe('/documents/doc-1?commentId=comment-2&annotationId=annot-3');
+	});
+
+	it('omits annotationId when null', () => {
+		const href = buildCommentHref('doc-1', 'comment-2', null);
+		expect(href).toBe('/documents/doc-1?commentId=comment-2');
+	});
+});

frontend/src/lib/utils/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;
+}