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__/actions/sprint-dates.test.ts

@@ -20,6 +20,11 @@ vi.mock('@/lib/prisma', () => ({
       create: vi.fn(),
       update: vi.fn(),
     },
+    user: {
+      findUnique: vi.fn().mockResolvedValue({ settings: {} }),
+      update: vi.fn().mockResolvedValue({}),
+    },
+    $executeRaw: vi.fn().mockResolvedValue(1),
   },
 }))
 

__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'
 

__tests__/components/split-pane.test.tsx

@@ -1,28 +1,35 @@
 // @vitest-environment jsdom
 import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'
 import { render, screen, fireEvent } from '@testing-library/react'
-import { SplitPane } from '@/components/split-pane/split-pane'
 
-// Helper to set a cookie
-function setCookie(key: string, value: string) {
-  Object.defineProperty(document, 'cookie', {
-    writable: true,
-    configurable: true,
-    value: `sp:${key}=${encodeURIComponent(value)}`,
+vi.mock('@/actions/user-settings', () => ({
+  updateUserSettingsAction: vi.fn().mockResolvedValue({ success: true, settings: {} }),
+}))
+
+import { SplitPane } from '@/components/split-pane/split-pane'
+import { useUserSettingsStore } from '@/stores/user-settings/store'
+
+function seedPositions(key: string, positions: number[]) {
+  useUserSettingsStore.setState((s) => {
+    s.entities.settings = {
+      layout: {
+        splitPanePositions: { [key]: positions },
+      },
+    }
   })
 }
 
-function clearCookies() {
-  Object.defineProperty(document, 'cookie', {
-    writable: true,
-    configurable: true,
-    value: '',
+function resetStore() {
+  useUserSettingsStore.setState((s) => {
+    s.entities.settings = {}
+    s.context.hydrated = false
+    s.context.isDemo = false
   })
 }
 
 describe('SplitPane', () => {
   beforeEach(() => {
-    clearCookies()
+    resetStore()
     // Default: desktop viewport
     Object.defineProperty(window, 'innerWidth', { writable: true, configurable: true, value: 1440 })
     window.dispatchEvent(new Event('resize'))
@@ -64,9 +71,8 @@ describe('SplitPane', () => {
     expect(dividers).toHaveLength(2)
   })
 
-  it('restores splits from cookie on mount', () => {
-    const stored = JSON.stringify([40, 60])
-    setCookie('test-restore', stored)
+  it('restores splits from user-settings store on mount', () => {
+    seedPositions('test-restore', [40, 60])
 
     const { container } = render(
       <SplitPane
@@ -81,8 +87,9 @@ describe('SplitPane', () => {
     expect(paneDiv).toBeTruthy()
   })
 
-  it('falls back to defaultSplit when cookie is invalid', () => {
-    setCookie('test-invalid', 'not-valid-json')
+  it('falls back to defaultSplit when persisted positions are invalid', () => {
+    // Wrong number of values for a 2-pane layout
+    seedPositions('test-invalid', [10, 30, 60])
 
     const { container } = render(
       <SplitPane