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/TrackMenu.test.ts

@@ -1,7 +1,7 @@
 import { afterEach, describe, expect, test, vi } from 'vitest';
 import { render, screen, fireEvent, waitFor } from '@testing-library/svelte';
-import { readable } from 'svelte/store';
 import { emptyLikesMock } from '../../test-utils/mocks/likes';
+import { emptyPlaylistsMock } from '../../test-utils/mocks/playlists';
 import { emptyQuarantineMock } from '../../test-utils/mocks/quarantine';
 import { makeTrack } from '$test-utils/fixtures/track';
 
@@ -29,12 +29,7 @@ vi.mock('$lib/api/admin/tracks', () => ({
   removeTrack: vi.fn().mockResolvedValue({ deleted_track_id: 't1' })
 }));
 
-vi.mock('$lib/api/playlists', () => ({
-  createPlaylistsQuery: () =>
-    readable({ data: { owned: [], public: [] }, isPending: false, isError: false }),
-  appendTracks: vi.fn().mockResolvedValue({ id: 'p1', tracks: [] }),
-  createPlaylist: vi.fn().mockResolvedValue({ id: 'p1', name: 'New' })
-}));
+vi.mock('$lib/api/playlists', () => emptyPlaylistsMock());
 
 vi.mock('$lib/player/store.svelte', () => ({
   playNext: vi.fn(),