test(PBI-76): align tests with new SplitPane and active-sprint flow

- split-pane.test.tsx: seed positions via Zustand store instead of
  document.cookie; mock @/actions/user-settings so the prisma client
  is not transitively initialised in jsdom.
- backlog-split-pane.test.tsx: same action mock.
- sprint-dates.test.ts: add user.findUnique/update + $executeRaw
  mocks because createSprintAction now writes to user-settings.

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

__tests__/components/backlog/backlog-split-pane.test.tsx

@@ -1,6 +1,11 @@
 // @vitest-environment jsdom
-import { describe, it, expect, beforeEach } from 'vitest'
+import { describe, it, expect, beforeEach, vi } from 'vitest'
 import { render, screen } from '@testing-library/react'
+
+vi.mock('@/actions/user-settings', () => ({
+  updateUserSettingsAction: vi.fn().mockResolvedValue({ success: true, settings: {} }),
+}))
+
 import { useProductWorkspaceStore } from '@/stores/product-workspace/store'
 import { BacklogSplitPane } from '@/components/backlog/backlog-split-pane'