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/lib/components/AlbumCard.test.ts

@@ -3,11 +3,10 @@ import { render, screen, fireEvent } from '@testing-library/svelte';
 import type { AlbumRef, AlbumDetail, TrackRef } from '$lib/api/types';
 import { FALLBACK_COVER } from '$lib/media/covers';
 import { emptyLikesMock } from '../../test-utils/mocks/likes';
+import { apiClientMock } from '../../test-utils/mocks/client';
 import { makeTrack } from '$test-utils/fixtures/track';
 
-vi.mock('$lib/api/client', () => ({
-  api: { get: vi.fn() }
-}));
+vi.mock('$lib/api/client', () => apiClientMock());
 vi.mock('$lib/player/store.svelte', () => ({
   enqueueTracks: vi.fn(),
   playQueue: vi.fn()