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/discover/discover.test.ts

@@ -1,6 +1,7 @@
 import { afterEach, beforeEach, describe, expect, test, vi } from 'vitest';
 import { render, screen, fireEvent, waitFor } from '@testing-library/svelte';
 import { mockQuery } from '../../test-utils/query';
+import { apiClientMock } from '../../test-utils/mocks/client';
 import type { LidarrSearchResult } from '$lib/api/types';
 
 // Lidarr search query factory and createRequest are mocked at the module
@@ -18,9 +19,7 @@ vi.mock('$lib/api/requests', () => ({
   createRequest: vi.fn().mockResolvedValue({ id: 'r1' })
 }));
 
-vi.mock('$lib/api/client', () => ({
-  api: { get: vi.fn(), post: vi.fn(), put: vi.fn(), del: vi.fn() }
-}));
+vi.mock('$lib/api/client', () => apiClientMock());
 
 import DiscoverPage from './+page.svelte';
 import { createLidarrSearchQuery } from '$lib/api/lidarr';