合并 AI 设计多会话客户端

整合 Enter 发送与多会话交互，保留服务端持久 Conversation Session，并补齐迁移、回归、Electron E2E 与 canonical 文档。

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;
 };
 
@@ -45,8 +48,10 @@ 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?(