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

electron/image-workspace/module.ts

@@ -2,6 +2,8 @@ import type {
   DesignAsset,
   DesignAssetUploadInput,
   DesignCapabilities,
+  DesignConversation,
+  DesignCreateConversationInput,
   DesignConfirmGenerationInput,
   DesignCreateWorkspaceInput,
   DesignGenerationTask,
@@ -19,6 +21,7 @@ export type DesignWorkspaceEventSubscription = {
 
 export type DesignWorkspaceEventSubscriptionInput = {
   workspaceId: string;
+  conversationId: string;
   afterEventId?: string;
 };
 
@@ -40,8 +43,10 @@ export interface DesignWorkspaceModule {
   createWorkspace(input: DesignCreateWorkspaceInput): Promise<DesignWorkspace>;
   renameWorkspace(input: DesignRenameWorkspaceInput): Promise<DesignWorkspace>;
   getWorkspace(workspaceId: string): Promise<DesignWorkspace>;
-  submitMessage(input: DesignSubmitMessageInput): Promise<DesignWorkspace>;
-  confirmGeneration(input: DesignConfirmGenerationInput): Promise<DesignWorkspace>;
+  createConversation(input: DesignCreateConversationInput): Promise<DesignConversation>;
+  getConversation(workspaceId: string, conversationId: string): Promise<DesignConversation>;
+  submitMessage(input: DesignSubmitMessageInput): Promise<DesignConversation>;
+  confirmGeneration(input: DesignConfirmGenerationInput): Promise<DesignConversation>;
   listTasks(workspaceId: string): Promise<DesignGenerationTask[]>;
   uploadAsset?(input: DesignAssetUploadInput): Promise<DesignAsset>;
   openWorkspaceEvents?(