feat: Day 14-17 - Frontend Chat Interface completed

Frontend:
- Create MessageList component with streaming animation
- Create MessageInput component with @knowledge base support
- Create ModelSelector component (DeepSeek/Qwen/Gemini)
- Implement conversationApi with SSE streaming
- Update AgentChatPage integrate all components
- Add Markdown rendering (react-markdown + remark-gfm)
- Add code highlighting (react-syntax-highlighter)
- Add vite-env.d.ts for environment variables

Features:
- Real-time streaming output with cursor animation
- Markdown and code block rendering
- Model switching (DeepSeek-V3, Qwen3-72B, Gemini Pro)
- @Knowledge base selector (UI ready)
- Auto-scroll to bottom
- Shift+Enter for new line, Enter to send
- Beautiful message bubble design

Build: Frontend build successfully (7.94s, 1.9MB)

New Files:
- components/chat/MessageList.tsx (170 lines)
- components/chat/MessageList.css (150 lines)
- components/chat/MessageInput.tsx (145 lines)
- components/chat/MessageInput.css (60 lines)
- components/chat/ModelSelector.tsx (110 lines)
- components/chat/ModelSelector.css (35 lines)
- api/conversationApi.ts (170 lines)
- src/vite-env.d.ts (9 lines)

Total: ~850 lines of new code

frontend/src/api/conversationApi.ts

@@ -0,0 +1,169 @@
+import request from './request';
+import type { ModelType } from '../components/chat/ModelSelector';
+
+export interface ApiResponse<T = any> {
+  success: boolean;
+  data?: T;
+  message?: string;
+}
+
+export interface Message {
+  id: string;
+  conversationId: string;
+  role: 'user' | 'assistant';
+  content: string;
+  model?: string;
+  metadata?: any;
+  tokens?: number;
+  createdAt: string;
+}
+
+export interface Conversation {
+  id: string;
+  userId: string;
+  projectId: string;
+  agentId: string;
+  title: string;
+  modelName: string;
+  messageCount: number;
+  totalTokens: number;
+  metadata?: any;
+  createdAt: string;
+  updatedAt: string;
+  project?: {
+    id: string;
+    name: string;
+    background?: string;
+    researchType?: string;
+  };
+  messages?: Message[];
+}
+
+export interface CreateConversationData {
+  projectId: string;
+  agentId: string;
+  title?: string;
+}
+
+export interface SendMessageData {
+  conversationId: string;
+  content: string;
+  modelType: ModelType;
+  knowledgeBaseIds?: string[];
+}
+
+export interface SendMessageResponse {
+  userMessage: Message;
+  assistantMessage: Message;
+  usage?: {
+    promptTokens: number;
+    completionTokens: number;
+    totalTokens: number;
+  };
+}
+
+// 创建对话
+export const createConversation = (data: CreateConversationData) => {
+  return request.post<ApiResponse<Conversation>>('/conversations', data);
+};
+
+// 获取对话列表
+export const getConversations = (projectId?: string) => {
+  const params = projectId ? { projectId } : {};
+  return request.get<ApiResponse<Conversation[]>>('/conversations', { params });
+};
+
+// 获取对话详情
+export const getConversationById = (id: string) => {
+  return request.get<ApiResponse<Conversation>>(`/conversations/${id}`);
+};
+
+// 发送消息（非流式）
+export const sendMessage = (data: SendMessageData) => {
+  return request.post<ApiResponse<SendMessageResponse>>('/conversations/message', data);
+};
+
+// 发送消息（流式输出）
+export const sendMessageStream = async (
+  data: SendMessageData,
+  onChunk: (content: string) => void,
+  onComplete: () => void,
+  onError: (error: Error) => void
+) => {
+  try {
+    const response = await fetch(`${import.meta.env.VITE_API_BASE_URL}/conversations/message/stream`, {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json',
+      },
+      body: JSON.stringify(data),
+    });
+
+    if (!response.ok) {
+      throw new Error(`HTTP error! status: ${response.status}`);
+    }
+
+    const reader = response.body?.getReader();
+    if (!reader) {
+      throw new Error('Response body is null');
+    }
+
+    const decoder = new TextDecoder();
+    let buffer = '';
+
+    while (true) {
+      const { done, value } = await reader.read();
+      
+      if (done) {
+        break;
+      }
+
+      buffer += decoder.decode(value, { stream: true });
+      const lines = buffer.split('\n');
+      buffer = lines.pop() || '';
+
+      for (const line of lines) {
+        const trimmedLine = line.trim();
+        if (!trimmedLine || !trimmedLine.startsWith('data:')) {
+          continue;
+        }
+
+        const dataStr = trimmedLine.slice(5).trim();
+        
+        if (dataStr === '[DONE]') {
+          onComplete();
+          return;
+        }
+
+        try {
+          const chunk = JSON.parse(dataStr);
+          if (chunk.content) {
+            onChunk(chunk.content);
+          }
+        } catch (parseError) {
+          console.error('Failed to parse SSE data:', parseError);
+        }
+      }
+    }
+
+    onComplete();
+  } catch (error) {
+    console.error('Stream error:', error);
+    onError(error as Error);
+  }
+};
+
+// 删除对话
+export const deleteConversation = (id: string) => {
+  return request.delete<ApiResponse>(`/conversations/${id}`);
+};
+
+export default {
+  createConversation,
+  getConversations,
+  getConversationById,
+  sendMessage,
+  sendMessageStream,
+  deleteConversation,
+};
+