feat: 5种专业登录页风格 + 外观设置 + 文档全面更新

- 新增 5 种登录页组件: SplitView/CenteredCard/SidePanel/FullImage/MinimalDark
- 提取 useLogin hook 共享登录逻辑
- LoginPage 改为风格分发器, localStorage(login-style) 控制渲染
- SettingsPage 新增外观设置 tab: 主题切换 + 登录风格选择器(含缩略图预览)
- AppearanceSettings 组件: 5 种风格卡片式切换
- 文档全面更新为 Go/Gin 架构:
  - ARCHITECTURE.md: Go 项目结构/handler/GORM 模型/中间件/LDAP/驱动矩阵
  - FEATURES.md: v0.3.1 功能清单含登录主题系统
  - ROADMAP.md: Python→Go 迁移历史 + Sprint 计划

frontend/src/hooks/useLogin.tsx

@@ -0,0 +1,38 @@
+import { useState, useCallback } from 'react';
+import { useNavigate } from 'react-router-dom';
+import { useAuth } from '@/hooks/useAuth';
+
+export interface LoginState {
+  username: string;
+  password: string;
+  loading: boolean;
+  error: string;
+  setUsername: (v: string) => void;
+  setPassword: (v: string) => void;
+  handleLogin: (e: React.FormEvent) => Promise<void>;
+}
+
+export function useLogin(): LoginState {
+  const navigate = useNavigate();
+  const { login } = useAuth();
+  const [username, setUsername] = useState('');
+  const [password, setPassword] = useState('');
+  const [loading, setLoading] = useState(false);
+  const [error, setError] = useState('');
+
+  const handleLogin = useCallback(async (e: React.FormEvent) => {
+    e.preventDefault();
+    if (!username || !password) return;
+    setLoading(true);
+    setError('');
+    const ok = await login(username, password);
+    setLoading(false);
+    if (ok) {
+      navigate('/dashboard');
+    } else {
+      setError('用户名或密码错误');
+    }
+  }, [username, password, login, navigate]);
+
+  return { username, password, loading, error, setUsername, setPassword, handleLogin };
+}