test(PBI-76): mock user-settings action in backlog integration test

PbiList now imports the user-settings store, which transitively
loads actions/user-settings.ts → lib/prisma. The vitest jsdom
environment has no DATABASE_URL, so we add a mock alongside the
existing action mocks.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

__tests__/components/backlog/integration.test.tsx

@@ -31,6 +31,9 @@ vi.mock('@/actions/stories', () => ({
 }))
 vi.mock('@/actions/pbis', () => ({ deletePbiAction: vi.fn().mockResolvedValue({ success: true }) }))
 vi.mock('@/actions/tasks', () => ({ reorderTasksAction: vi.fn().mockResolvedValue({ success: true }) }))
+vi.mock('@/actions/user-settings', () => ({
+  updateUserSettingsAction: vi.fn().mockResolvedValue({ success: true, settings: {} }),
+}))
 vi.mock('sonner', () => ({ toast: { error: vi.fn(), success: vi.fn() } }))
 
 // Mock dnd-kit