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/albums/page.test.ts

@@ -2,6 +2,7 @@ import { describe, expect, test, vi } from 'vitest';
 import { render, screen } from '@testing-library/svelte';
 import { readable } from 'svelte/store';
 import { emptyLikesMock } from '../../../test-utils/mocks/likes';
+import { apiClientMock } from '../../../test-utils/mocks/client';
 
 vi.mock('$lib/api/albums', () => ({
   createAlbumsAlphaInfiniteQuery: () => readable({
@@ -25,7 +26,7 @@ vi.mock('$lib/api/albums', () => ({
   }),
   ALBUM_PAGE_SIZE: 50
 }));
-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()