refactor(web): add test-utils helpers for likes/quarantine/track/appState mocks (#375)

Pre-sweep checkpoint. Adds:
- fixtures/track.ts        — makeTrack() / makeTracks(n) (18 files duplicate today)
- mocks/likes.ts           — emptyLikesMock() (19 files duplicate today)
- mocks/quarantine.ts      — emptyQuarantineMock() (9 files duplicate today)
- mocks/appState.ts        — pageUrlModule(state) (8 files duplicate today)

The sweep commits that follow update test files to use these helpers.

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

web/src/test-utils/fixtures/track.ts

@@ -0,0 +1,31 @@
+// Fixture builders for TrackRef. Use in test setup to avoid
+// re-declaring the same literal across 18+ files.
+
+import type { TrackRef } from '$lib/api/types';
+
+export function makeTrack(overrides: Partial<TrackRef> = {}): TrackRef {
+  return {
+    id: 't1',
+    title: 'Test Track',
+    album_id: 'a1',
+    album_title: 'Test Album',
+    artist_id: 'ar1',
+    artist_name: 'Test Artist',
+    track_number: 1,
+    disc_number: 1,
+    duration_sec: 180,
+    stream_url: '/api/tracks/t1/stream',
+    ...overrides
+  };
+}
+
+export function makeTracks(n: number, overrides: Partial<TrackRef> = {}): TrackRef[] {
+  return Array.from({ length: n }, (_, i) =>
+    makeTrack({
+      id: `t${i + 1}`,
+      title: `Test Track ${i + 1}`,
+      stream_url: `/api/tracks/t${i + 1}/stream`,
+      ...overrides
+    })
+  );
+}