feat: 添加会话历史管理功能

- 在 AI 教练聊天界面中新增会话历史查看和选择功能，用户可以查看和选择之前的会话记录
- 实现会话删除功能，用户可以删除不需要的会话记录
- 优化历史会话的加载和展示，提升用户体验
- 更新相关样式以适应新功能的展示和交互

services/aiCoach.ts

@@ -0,0 +1,38 @@
+import { api } from '@/services/api';
+
+export type AiConversationListItem = {
+  conversationId: string;
+  title?: string | null;
+  lastMessageAt?: string | null;
+  createdAt?: string | null;
+};
+
+export type AiConversationListResponse = {
+  page: number;
+  pageSize: number;
+  total: number;
+  items: AiConversationListItem[];
+};
+
+export type AiConversationDetail = {
+  conversationId: string;
+  title?: string | null;
+  lastMessageAt?: string | null;
+  createdAt?: string | null;
+  messages: Array<{ role: 'user' | 'assistant' | 'system'; content: string; createdAt?: string | null }>;
+};
+
+export async function listConversations(page = 1, pageSize = 20): Promise<AiConversationListResponse> {
+  const qs = `?page=${encodeURIComponent(page)}&pageSize=${encodeURIComponent(pageSize)}`;
+  return api.get<AiConversationListResponse>(`/api/ai-coach/conversations${qs}`);
+}
+
+export async function getConversationDetail(conversationId: string): Promise<AiConversationDetail> {
+  return api.get<AiConversationDetail>(`/api/ai-coach/conversations/${encodeURIComponent(conversationId)}`);
+}
+
+export async function deleteConversation(conversationId: string): Promise<{ success: boolean }> {
+  return api.delete<{ success: boolean }>(`/api/ai-coach/conversations/${encodeURIComponent(conversationId)}`);
+}
+
+