feat: 完成多用户系统（登录/注册界面 + 用户隔离）

frontend-react/src/index.css

@@ -262,6 +262,128 @@ header p {
   color: #999;
 }
 
+/* 认证界面 */
+.auth-container {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  min-height: 100vh;
+  padding: 20px;
+}
+
+.auth-card {
+  background: white;
+  padding: 40px;
+  border-radius: 10px;
+  box-shadow: 0 4px 20px rgba(0,0,0,0.2);
+  width: 100%;
+  max-width: 400px;
+}
+
+.auth-card h1 {
+  color: #667eea;
+  font-size: 2em;
+  text-align: center;
+  margin-bottom: 10px;
+}
+
+.auth-card h2 {
+  color: #333;
+  font-size: 1.5em;
+  text-align: center;
+  margin-bottom: 30px;
+}
+
+.form-group {
+  margin-bottom: 20px;
+}
+
+.form-group label {
+  display: block;
+  color: #555;
+  font-weight: 500;
+  margin-bottom: 8px;
+}
+
+.form-group input {
+  width: 100%;
+  padding: 12px;
+  border: 1px solid #ddd;
+  border-radius: 5px;
+  font-size: 1em;
+  transition: border-color 0.2s;
+}
+
+.form-group input:focus {
+  outline: none;
+  border-color: #667eea;
+}
+
+.btn-primary {
+  width: 100%;
+  padding: 12px;
+  background: linear-gradient(135deg, #667eea, #764ba2);
+  color: white;
+  border: none;
+  border-radius: 5px;
+  font-size: 1.1em;
+  font-weight: 600;
+  cursor: pointer;
+  transition: transform 0.2s;
+}
+
+.btn-primary:hover {
+  transform: translateY(-2px);
+}
+
+.btn-primary:disabled {
+  opacity: 0.6;
+  cursor: not-allowed;
+}
+
+.auth-tip {
+  text-align: center;
+  margin-top: 20px;
+  color: #666;
+}
+
+.auth-tip a {
+  color: #667eea;
+  text-decoration: none;
+}
+
+.auth-tip a:hover {
+  text-decoration: underline;
+}
+
+/* 用户信息 */
+.header-content {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+}
+
+.user-info {
+  display: flex;
+  align-items: center;
+  gap: 15px;
+  color: white;
+}
+
+.btn-logout {
+  padding: 8px 16px;
+  background: rgba(255,255,255,0.2);
+  color: white;
+  border: 1px solid rgba(255,255,255,0.5);
+  border-radius: 5px;
+  cursor: pointer;
+  transition: background 0.2s;
+}
+
+.btn-logout:hover {
+  background: rgba(255,255,255,0.3);
+}
+
 /* 移动端 */
 @media (max-width: 768px) {
   .grid-2 {