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/mocks/likes.ts

@@ -0,0 +1,25 @@
+// Default empty-LikedIds mock for $lib/api/likes. Mirrors the
+// TanStack Query readable shape (data + isPending + isError) that
+// 19 test files were duplicating verbatim.
+//
+// Usage:
+//   vi.mock('$lib/api/likes', () => emptyLikesMock());
+//
+// Tests that assert on liked-state changes (LikeButton.test.ts) keep
+// their state-driven mocks inline.
+
+import { readable } from 'svelte/store';
+import { vi } from 'vitest';
+
+export function emptyLikesMock() {
+  return {
+    createLikedIdsQuery: () =>
+      readable({
+        data: { track_ids: [], album_ids: [], artist_ids: [] },
+        isPending: false,
+        isError: false
+      }),
+    likeEntity: vi.fn().mockResolvedValue(undefined),
+    unlikeEntity: vi.fn().mockResolvedValue(undefined)
+  };
+}