🎨 更新 React 登录界面：多身份选择

功能:
- 身份模式选择（人类/龙虾/双重）
- 龙虾选择下拉框
- 自动扫描本机龙虾
- 登录时保存 sessions 信息

界面:
- 单选按钮选择身份模式
- 根据模式显示/隐藏龙虾选择
- 显示龙虾列表（从实例扫描）

frontend/src/App.js

@@ -14,17 +14,49 @@ axios.interceptors.request.use(config => {
 function LoginPage() {
   const [username, setUsername] = useState('test');
   const [password, setPassword] = useState('test123');
+  const [loginMode, setLoginMode] = useState('human_only');
+  const [agents, setAgents] = useState([]);
+  const [selectedAgent, setSelectedAgent] = useState('');
   const navigate = useNavigate();
 
+  // 扫描本机龙虾
+  useEffect(() => {
+    scanAgents();
+  }, []);
+
+  const scanAgents = async () => {
+    try {
+      const res = await axios.get(`${API_BASE}/user/scan-local-agents/`);
+      setAgents(res.data.agents || []);
+      if (res.data.agents?.length > 0) {
+        setSelectedAgent(res.data.agents[0].agent_id);
+      }
+    } catch (error) {
+      console.error('扫描龙虾失败:', error);
+    }
+  };
+
   const handleLogin = async (e) => {
     e.preventDefault();
     try {
-      const res = await axios.post(`${API_BASE}/auth/login/`, { username, password });
+      const payload = {
+        username,
+        password,
+        login_mode: loginMode
+      };
+      
+      if (loginMode !== 'human_only' && selectedAgent) {
+        payload.selected_agent_id = selectedAgent;
+      }
+      
+      const res = await axios.post(`${API_BASE}/auth/login/`, payload);
       localStorage.setItem('token', res.data.token);
       localStorage.setItem('user', JSON.stringify(res.data.user));
+      localStorage.setItem('sessions', JSON.stringify(res.data.sessions));
+      localStorage.setItem('login_mode', res.data.login_mode);
       navigate('/meetings');
     } catch (error) {
-      alert('登录失败：' + (error.response?.data?.detail || error.message));
+      alert('登录失败：' + (error.response?.data?.detail || error.response?.data?.error || error.message));
     }
   };
 
@@ -32,9 +64,67 @@ function LoginPage() {
     <div style={styles.center}>
       <div style={styles.card}>
         <h1 style={styles.title}>🏛️ 龙虾议事厅</h1>
-        <form onSubmit={handleLogin} style={styles.form}>
+        <form onSubmit={handleLogin} style={{...styles.form, flexDirection: 'column'}}>
           <input type="text" placeholder="用户名" value={username} onChange={e => setUsername(e.target.value)} style={styles.input} required />
           <input type="password" placeholder="密码" value={password} onChange={e => setPassword(e.target.value)} style={styles.input} required />
+          
+          {/* 身份模式选择 */}
+          <div style={{margin: '15px 0'}}>
+            <label style={{display: 'block', marginBottom: '10px', fontWeight: '600'}}>登录身份：</label>
+            <label style={{display: 'block', marginBottom: '8px', cursor: 'pointer'}}>
+              <input 
+                type="radio" 
+                name="loginMode" 
+                value="human_only" 
+                checked={loginMode === 'human_only'} 
+                onChange={e => setLoginMode(e.target.value)} 
+              />
+              {' '}👤 人类身份（纯人类参会）
+            </label>
+            <label style={{display: 'block', marginBottom: '8px', cursor: 'pointer'}}>
+              <input 
+                type="radio" 
+                name="loginMode" 
+                value="agent_only" 
+                checked={loginMode === 'agent_only'} 
+                onChange={e => setLoginMode(e.target.value)} 
+              />
+              {' '}🦞 龙虾身份（Agent 参会）
+            </label>
+            <label style={{display: 'block', marginBottom: '8px', cursor: 'pointer'}}>
+              <input 
+                type="radio" 
+                name="loginMode" 
+                value="both" 
+                checked={loginMode === 'both'} 
+                onChange={e => setLoginMode(e.target.value)} 
+              />
+              {' '}👤+ 双重身份（人类 + 龙虾）
+            </label>
+          </div>
+          
+          {/* 龙虾选择 */}
+          {loginMode !== 'human_only' && (
+            <div style={{margin: '15px 0'}}>
+              <label style={{display: 'block', marginBottom: '8px', fontWeight: '600'}}>选择龙虾：</label>
+              <select 
+                value={selectedAgent} 
+                onChange={e => setSelectedAgent(e.target.value)}
+                style={styles.input}
+              >
+                {agents.length === 0 ? (
+                  <option value="">未找到龙虾</option>
+                ) : (
+                  agents.map(a => (
+                    <option key={a.agent_id} value={a.agent_id}>
+                      {a.agent_id} ({a.instance_name})
+                    </option>
+                  ))
+                )}
+              </select>
+            </div>
+          )}
+          
           <button type="submit" style={styles.btn}>登录</button>
         </form>
       </div>