test: add AnnotationSidePanel spec and fix env mock in layout spec

- AnnotationSidePanel: cover visibility (null vs set annotationId),
  close button callback, and targetCommentId forwarding
- layout.svelte.spec: mock $env/static/public to satisfy
  PUBLIC_NOTIFICATION_POLL_MS import from NotificationBell
- mention.spec: update assertion to match span-based mention rendering

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

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

@@ -96,6 +96,7 @@ describe('renderBody', () => {
 		const mentions: MentionDTO[] = [{ id: 'uuid-1', firstName: 'Hans', lastName: 'Müller' }];
 		const result = renderBody('Hey @Hans Müller!', mentions);
 		expect(result).toContain('<span');
+		expect(result).toContain('class="mention"');
 		expect(result).toContain('Hans Müller');
 	});