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/test-utils/mocks/client.ts

@@ -0,0 +1,22 @@
+// Default api-client mock for $lib/api/client. Provides spies for
+// every HTTP verb so tests don't need to anticipate which methods
+// the code under test might reach for.
+//
+// Usage:
+//   vi.mock('$lib/api/client', () => apiClientMock());
+//
+// Tests that need a specific resolved value (e.g. events tests that
+// assert on the play_event_id round trip) keep custom mocks inline.
+
+import { vi } from 'vitest';
+
+export function apiClientMock() {
+  return {
+    api: {
+      get: vi.fn(),
+      post: vi.fn(),
+      put: vi.fn(),
+      del: vi.fn()
+    }
+  };
+}

web/src/test-utils/mocks/playlists.ts

@@ -0,0 +1,23 @@
+// Default empty-playlists mock for $lib/api/playlists. The
+// dominant shape across the menu / card tests: no playlists for
+// the user, append/create as resolved spies.
+//
+// Usage:
+//   vi.mock('$lib/api/playlists', () => emptyPlaylistsMock());
+//
+// Tests that exercise richer return payloads (AddToPlaylistMenu's
+// full PlaylistDetail return, PlaylistCard's getPlaylist, route
+// tests that need owned playlists pre-populated) keep custom
+// mocks inline.
+
+import { readable } from 'svelte/store';
+import { vi } from 'vitest';
+
+export function emptyPlaylistsMock() {
+  return {
+    createPlaylistsQuery: () =>
+      readable({ data: { owned: [], public: [] }, isPending: false, isError: false }),
+    appendTracks: vi.fn().mockResolvedValue({ id: 'p1', tracks: [] }),
+    createPlaylist: vi.fn().mockResolvedValue({ id: 'p1', name: 'New' })
+  };
+}