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

shared/image-workspace.ts

@@ -47,14 +47,28 @@ export type DesignMessage = {
 export type DesignWorkspaceSummary = {
   workspaceId: string;
   title: string;
-  turnRevision: number;
   viewRevision: number;
+  conversationCount: number;
   phase: DesignWorkspacePhase;
-  brief: DesignBrief;
   updatedAt: string;
 };
 
 export type DesignWorkspace = DesignWorkspaceSummary & {
+  conversations: DesignConversationSummary[];
+};
+
+export type DesignConversationSummary = {
+  conversationId: string;
+  workspaceId: string;
+  title: string;
+  turnRevision: number;
+  phase: DesignWorkspacePhase;
+  brief: DesignBrief;
+  createdAt: string;
+  updatedAt: string;
+};
+
+export type DesignConversation = DesignConversationSummary & {
   messages: DesignMessage[];
 };
 
@@ -86,6 +100,7 @@ export type DesignAssetUploadInput = {
 export type DesignGenerationTask = {
   taskId: string;
   workspaceId: string;
+  conversationId: string | null;
   medium: DesignMedium;
   status: DesignTaskStatus;
   briefVersion: number;
@@ -106,12 +121,13 @@ export type DesignGenerationTaskUpdatedEvent = {
   generationTask: DesignGenerationTask;
 };
 
-export type DesignGenerationTasksSnapshotEvent = {
+export type DesignConversationSnapshotEvent = {
   id: string;
-  type: 'design.generation_tasks.snapshot';
+  type: 'design.conversation.snapshot';
   workspaceId: string;
+  conversationId: string;
   workspaceViewRevision: number;
-  workspace: DesignWorkspace;
+  conversation: DesignConversation;
   generationTasks: DesignGenerationTask[];
 };
 
@@ -119,6 +135,7 @@ export type DesignAssistantDeltaEvent = {
   id: string;
   type: 'design.assistant.delta';
   workspaceId: string;
+  conversationId: string;
   clientTurnId: string;
   turnRevision: number;
   chunkIndex: number;
@@ -127,7 +144,7 @@ export type DesignAssistantDeltaEvent = {
 
 export type DesignWorkspaceEvent =
   | DesignGenerationTaskUpdatedEvent
-  | DesignGenerationTasksSnapshotEvent
+  | DesignConversationSnapshotEvent
   | DesignAssistantDeltaEvent;
 
 export type DesignWorkspaceBootstrap = {
@@ -140,6 +157,12 @@ export type DesignCreateWorkspaceInput = {
   title: string;
 };
 
+export type DesignCreateConversationInput = {
+  workspaceId: string;
+  clientConversationId: string;
+  title: string;
+};
+
 export type DesignRenameWorkspaceInput = {
   workspaceId: string;
   title: string;
@@ -147,6 +170,7 @@ export type DesignRenameWorkspaceInput = {
 
 export type DesignSubmitMessageInput = {
   workspaceId: string;
+  conversationId: string;
   clientTurnId: string;
   expectedTurnRevision: number;
   message: string;
@@ -155,6 +179,7 @@ export type DesignSubmitMessageInput = {
 
 export type DesignConfirmGenerationInput = {
   workspaceId: string;
+  conversationId: string;
   clientTurnId: string;
   expectedTurnRevision: number;
   quoteId: string;