test: fix the Suspect sources tests CI rejected (#5410)

- staticcheck QF1001: the nil-vs-empty check in TestSourceMarkersFor is
  now an early continue instead of a negated conjunction.
- The page test mocks #lib/api/admin.js whole, so suspectSourcesNextOffset
  was undefined there. Its test moves to admin.suspect-sources.test.ts,
  beside admin.scan.test.ts, against the real module.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/lib/api/admin.suspect-sources.test.ts

@@ -0,0 +1,43 @@
+import { describe, expect, test } from 'vitest';
+import { suspectSourcesNextOffset } from './admin';
+import type { AdminSuspectResponse, AdminSuspectTrack } from './types';
+
+function track(id: string): AdminSuspectTrack {
+  return {
+    track_id: id,
+    title: id,
+    artist_id: 'ar',
+    artist_name: 'Artist',
+    album_id: 'al',
+    album_title: 'Album',
+    file_path: `/music/${id}.mp3`,
+    duration_sec: 180,
+    disc_number: null,
+    track_number: null,
+    markers: []
+  };
+}
+
+function page(offset: number, total: number): AdminSuspectResponse {
+  return {
+    total,
+    limit: 50,
+    offset,
+    groups: [
+      { directory: 'a', tracks: [track('1'), track('2')] },
+      { directory: 'b', tracks: [track('3')] }
+    ]
+  };
+}
+
+// limit and total are in tracks, so the next offset must count tracks across
+// groups, not the number of groups.
+describe('suspectSourcesNextOffset', () => {
+  test('counts tracks, not groups', () => {
+    expect(suspectSourcesNextOffset(page(0, 10))).toBe(3);
+  });
+
+  test('stops at the total', () => {
+    expect(suspectSourcesNextOffset(page(7, 10))).toBeUndefined();
+  });
+});