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/routes/layout.svelte.spec.ts

@@ -1,8 +1,10 @@
-import { afterEach, describe, expect, it } from 'vitest';
+import { afterEach, describe, expect, it, vi } from 'vitest';
 import { cleanup, render } from 'vitest-browser-svelte';
 import { page, userEvent } from 'vitest/browser';
 import { createRawSnippet } from 'svelte';
 
+vi.mock('$env/static/public', () => ({ PUBLIC_NOTIFICATION_POLL_MS: '60000' }));
+
 afterEach(cleanup);
 
 const emptySnippet = createRawSnippet(() => ({ render: () => '<span></span>' }));