feat: 支持 AI 设计项目多会话交互

需求：用户可在同一设计项目中新建和切换会话，任务列表保持项目级共享。

实现：
- 增加会话选择、新建入口及本地数据迁移
- Conversation 独立消息、Brief、Quote 与流式状态
- 保留项目任务并修复 Task revision 与 ABA 异步竞态
- 保持 Enter 发送、Shift+Enter 换行及 IME 保护

tests/unit/local-image-workspace.test.ts

@@ -87,7 +87,7 @@ describe('local AI design workspace', () => {
 
     const reloaded = new LocalImageWorkspace({ userDataDir });
     await expect(reloaded.bootstrap()).resolves.toMatchObject({
-      workspaces: [{ title: '角色设计第二版', turnRevision: 0, viewRevision: 1 }],
+      workspaces: [{ title: '角色设计第二版', conversationCount: 1, viewRevision: 1 }],
     });
   });
 
@@ -97,8 +97,10 @@ describe('local AI design workspace', () => {
       clientWorkspaceId: 'client-flow',
       title: '海洋公益海报',
     });
+    const conversationId = created.conversations[0].conversationId;
     const discussed = await service.submitMessage({
       workspaceId: created.workspaceId,
+      conversationId,
       clientTurnId: 'turn-one',
       expectedTurnRevision: 0,
       message: '做一张保护海洋的竖版公益海报',
@@ -111,6 +113,7 @@ describe('local AI design workspace', () => {
 
     const confirmed = await service.confirmGeneration({
       workspaceId: created.workspaceId,
+      conversationId,
       clientTurnId: 'turn-two',
       expectedTurnRevision: 1,
       quoteId: quote!.quoteId,
@@ -141,15 +144,17 @@ describe('local AI design workspace', () => {
       clientWorkspaceId: 'client-reset',
       title: '可重置项目',
     });
+    const conversationId = created.conversations[0].conversationId;
 
     await expect(service.submitMessage({
       workspaceId: created.workspaceId,
+      conversationId,
       clientTurnId: 'stale-turn',
       expectedTurnRevision: 3,
       message: '测试',
     })).rejects.toMatchObject<LocalImageWorkspaceError>({
       status: 409,
-      code: 'workspace_revision_conflict',
+      code: 'conversation_revision_conflict',
     });
 
     expect(existsSync(getLocalImageWorkspaceDirectory(userDataDir))).toBe(true);