feat: 添加起始日期功能并更新相关组件与测试用例

- 在任务创建和编辑界面中新增起始日期字段，默认值为当天。
- 更新任务卡片展示逻辑，仅在起始日期不是当天时显示相关信息。
- 修改相关测试用例以覆盖起始日期的功能，包括批量创建和编辑任务的场景。
- 数据库模式版本更新至4，以支持起始日期字段的存储与迁移。

src/data/schema.spec.ts

@@ -21,6 +21,7 @@ function validTask(overrides: Record<string, unknown> = {}): Record<string, unkn
     priority: 2,
     tags: ['设计'],
     dueDate: '2026-10-20',
+    startDate: null,
     sortOrder: 1024,
     createdAt: '2026-10-18T00:00:00.000Z',
     updatedAt: '2026-10-18T00:00:00.000Z',
@@ -97,6 +98,12 @@ describe('coerceTask', () => {
     expect(result?.sortOrder).toBe(0)
   })
 
+  it('合法 startDate 保留，非法 startDate 收敛为 null', () => {
+    expect(coerceTask(validTask({ startDate: '2026-10-01' }))?.startDate).toBe('2026-10-01')
+    expect(coerceTask(validTask({ startDate: '2026-02-31' }))?.startDate).toBeNull()
+    expect(coerceTask(validTask({ startDate: undefined }))?.startDate).toBeNull()
+  })
+
   it('tags 过滤掉非字符串与空白项', () => {
     const result = coerceTask(validTask({ tags: ['ok', 1, null, '  ', 'ok2'] }))
     expect(result?.tags).toEqual(['ok', 'ok2'])
@@ -265,17 +272,46 @@ describe('版本迁移', () => {
     expect(state.tasks[0]?.archivedAt).toBe('2026-01-03T00:00:00.000Z')
   })
 
-  it('v0/v1/v2 都能一路迁移到 v3', () => {
+  it('v3 数据迁移到 v4 时补上 startDate，其余字段不变', () => {
+    const v3 = JSON.stringify({
+      schemaVersion: 3,
+      tasks: [
+        {
+          id: 'legacy',
+          title: '旧任务',
+          status: 'todo',
+          priority: 2,
+          sortOrder: 1024,
+          createdAt: '2026-01-01T00:00:00.000Z',
+          updatedAt: '2026-01-01T00:00:00.000Z',
+          completedAt: null,
+          archivedAt: null,
+          deletedAt: null,
+        },
+      ],
+    })
+
+    const state = parsePersistedState(v3)
+
+    expect(state.schemaVersion).toBe(SCHEMA_VERSION)
+    expect(state.tasks[0]?.startDate).toBeNull()
+    expect(state.tasks[0]?.deletedAt).toBeNull()
+    expect(state.tasks[0]?.title).toBe('旧任务')
+  })
+
+  it('v0/v1/v2/v3 都能一路迁移到当前版本', () => {
     const builds = [
       () => ({ tasks: [{ id: 'a', title: 'A', status: 'todo', priority: 2 }] }),
       () => ({ schemaVersion: 1, tasks: [{ id: 'a', title: 'A', status: 'todo', priority: 2 }] }),
       () => ({ schemaVersion: 2, tasks: [{ id: 'a', title: 'A', status: 'todo', priority: 2 }] }),
+      () => ({ schemaVersion: 3, tasks: [{ id: 'a', title: 'A', status: 'todo', priority: 2 }] }),
     ]
 
     for (const build of builds) {
       const state = parsePersistedState(JSON.stringify(build()))
-      expect(state.schemaVersion).toBe(3)
+      expect(state.schemaVersion).toBe(SCHEMA_VERSION)
       expect(state.tasks[0]?.deletedAt).toBeNull()
+      expect(state.tasks[0]?.startDate).toBeNull()
     }
   })