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/components/login/CenteredCard.tsx

@@ -0,0 +1,61 @@
+import { useLogin } from '@/hooks/useLogin';
+import { useTheme } from '@/hooks/useTheme';
+import { Sun, Moon } from 'lucide-react';
+
+export default function CenteredCard() {
+  const { username, password, loading, error, setUsername, setPassword, handleLogin } = useLogin();
+  const { toggleTheme, isDark } = useTheme();
+
+  return (
+    <div style={{ minHeight: '100vh', display: 'flex', alignItems: 'center', justifyContent: 'center', padding: 24, position: 'relative' }}>
+      {/* Gradient background */}
+      <div className="dg-gradient-br" style={{ position: 'fixed', inset: 0, zIndex: 0 }} />
+
+      {/* Theme toggle */}
+      <button
+        onClick={toggleTheme}
+        style={{ position: 'fixed', top: 20, right: 20, zIndex: 50, background: 'rgba(255,255,255,0.1)', border: 'none', borderRadius: 8, padding: 8, cursor: 'pointer', color: '#fff', backdropFilter: 'blur(4px)' }}
+      >
+        {isDark ? <Sun size={18} /> : <Moon size={18} />}
+      </button>
+
+      {/* Card */}
+      <div style={{ position: 'relative', zIndex: 10, width: '100%', maxWidth: 400, background: 'var(--dg-bg-primary)', borderRadius: 14, padding: '40px 32px', boxShadow: '0 8px 32px rgba(0,0,0,0.3)' }}>
+        <div style={{ textAlign: 'center', marginBottom: 32 }}>
+          <h1 style={{ fontSize: 24, fontWeight: 700, marginBottom: 6 }}>
+            <span className="dg-gradient-text">SQL Audit Platform</span>
+          </h1>
+          <p style={{ color: 'var(--dg-text-muted)', fontSize: 14 }}>Sign in to your account</p>
+        </div>
+
+        <form onSubmit={handleLogin} style={{ display: 'flex', flexDirection: 'column', gap: 16 }}>
+          {error && <div style={{ color: '#f87171', fontSize: 13, textAlign: 'center' }}>{error}</div>}
+
+          <input
+            type="text"
+            placeholder="Username"
+            value={username}
+            onChange={(e) => setUsername(e.target.value)}
+            style={{ padding: '11px 14px', borderRadius: 8, background: 'var(--dg-bg-secondary)', border: '1px solid var(--dg-border)', color: 'var(--dg-text-primary)', fontSize: 14, outline: 'none' }}
+          />
+          <input
+            type="password"
+            placeholder="Password"
+            value={password}
+            onChange={(e) => setPassword(e.target.value)}
+            style={{ padding: '11px 14px', borderRadius: 8, background: 'var(--dg-bg-secondary)', border: '1px solid var(--dg-border)', color: 'var(--dg-text-primary)', fontSize: 14, outline: 'none' }}
+          />
+
+          <button
+            type="submit"
+            disabled={loading || !username || !password}
+            className="dg-gradient-bg"
+            style={{ padding: '11px 14px', borderRadius: 8, border: 'none', color: '#fff', fontWeight: 600, fontSize: 14, cursor: loading || !username || !password ? 'not-allowed' : 'pointer', opacity: loading || !username || !password ? 0.6 : 1, marginTop: 4 }}
+          >
+            {loading ? 'Signing in…' : 'Sign in'}
+          </button>
+        </form>
+      </div>
+    </div>
+  );
+}