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>

internal/api/admin_suspect_sources_test.go

@@ -39,7 +39,10 @@ func TestSourceMarkersFor(t *testing.T) {
 	}
 	for _, c := range cases {
 		got := sourceMarkersFor("/music/x/" + c.base)
-		if !slices.Equal(got, c.want) && !(len(got) == 0 && len(c.want) == 0) {
+		if len(got) == 0 && len(c.want) == 0 {
+			continue // nil and empty both mean "no markers"
+		}
+		if !slices.Equal(got, c.want) {
 			t.Errorf("%s:\n  got  %v\n  want %v", c.base, got, c.want)
 		}
 	}