refactor(web): extract emptyPlaylistsMock + apiClientMock test helpers

The DRY pass audit (#375) found two inline mock patterns repeated across the
vitest suite: a default empty-playlists mock for $lib/api/playlists (4 exact
copies in menu/card tests) and an api-client spy mock for $lib/api/client
(9 callers split between get-only and full-RESTy shapes — unified into one
helper that always returns all four verb spies).

Mirrors the existing test-utils/mocks/likes.ts and test-utils/mocks/quarantine.ts
convention. Tests with intentionally divergent shapes (AddToPlaylistMenu's
richer createPlaylist payload, PlaylistCard's getPlaylist, route-specific
mocks, events.svelte's specific resolved value) stay inline.

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

web/src/routes/library/liked/liked.test.ts

@@ -1,6 +1,7 @@
 import { afterEach, describe, expect, test, vi } from 'vitest';
 import { render, screen, fireEvent } from '@testing-library/svelte';
 import { mockInfiniteQuery } from '../../../test-utils/query';
+import { apiClientMock } from '../../../test-utils/mocks/client';
 import type { ArtistRef, AlbumRef, TrackRef, Page } from '$lib/api/types';
 import { readable } from 'svelte/store';
 import { makeTrack } from '$test-utils/fixtures/track';
@@ -18,7 +19,7 @@ vi.mock('$lib/api/likes', () => ({
   unlikeEntity: vi.fn()
 }));
 
-vi.mock('$lib/api/client', () => ({ api: { get: vi.fn() } }));
+vi.mock('$lib/api/client', () => apiClientMock());
 vi.mock('$lib/player/store.svelte', () => ({
   playQueue: vi.fn(), enqueueTrack: vi.fn(), enqueueTracks: vi.fn()
 }));