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/auth/store.test.ts

@@ -1,12 +1,7 @@
 import { afterEach, beforeEach, describe, expect, test, vi } from 'vitest';
+import { apiClientMock } from '../../test-utils/mocks/client';
 
-vi.mock('$lib/api/client', () => ({
-  api: {
-    get: vi.fn(),
-    post: vi.fn(),
-    del: vi.fn()
-  }
-}));
+vi.mock('$lib/api/client', () => apiClientMock());
 
 vi.mock('$lib/query/client', () => ({
   queryClient: { clear: vi.fn() }