feat: 查询页增强 + 权限体系优化

- Redis DB0~DB15 下拉选择，默认DB0
- 查询历史粘贴按钮（一键填入SQL编辑器）
- 去掉导出历史按钮
- SchemaSidebar 按目标数据库筛选
- SQL自动补全 schema 双层注册（db→tables, table→columns）
- 后端 db-schema 接口（INFORMATION_SCHEMA.COLUMNS 一次加载）
- autocompletion 扩展激活
- Redis key 树形结构（冒号分隔层级）
- Redis 查询模板 GET/HGETALL/HGET
- guest 角色彻底移除，改为 auditor
- 登录页优化：去掉演示账号、placeholder 中文提示

frontend/src/pages/LoginPage.tsx

@@ -92,12 +92,12 @@ export default function LoginPage() {
 
           <form onSubmit={handleLogin} className="space-y-4">
             <div>
-              <label className="block text-sm font-medium mb-2">用户名 / 邮箱</label>
+              <label className="block text-sm font-medium mb-2">用户名</label>
               <div className="relative">
                 <UserCog className="absolute left-3.5 top-1/2 -translate-y-1/2 w-4 h-4 text-[var(--dg-text-muted)]" />
                 <input
                   className="w-full pl-10 pr-4 py-3 bg-[var(--dg-bg-tertiary)] border border-[var(--dg-border)] rounded-lg text-sm focus:border-blue-400 focus:outline-none focus:ring-1 focus:ring-blue-400/20 transition-all"
-                  placeholder="admin"
+                  placeholder="请输入用户名"
                   value={username}
                   onChange={e => setUsername(e.target.value)}
                 />
@@ -110,7 +110,7 @@ export default function LoginPage() {
                 <input
                   type="password"
                   className="w-full pl-10 pr-4 py-3 bg-[var(--dg-bg-tertiary)] border border-[var(--dg-border)] rounded-lg text-sm focus:border-blue-400 focus:outline-none focus:ring-1 focus:ring-blue-400/20 transition-all"
-                  placeholder="123456"
+                  placeholder="请输入密码"
                   value={password}
                   onChange={e => setPassword(e.target.value)}
                 />
@@ -127,18 +127,6 @@ export default function LoginPage() {
             </button>
             {error && <p className="text-xs text-red-400 text-center mt-2">{error}</p>}
           </form>
-
-          <div className="mt-6 p-3 rounded-lg bg-[var(--dg-bg-tertiary)] border border-[var(--dg-border)]">
-            <p className="text-xs text-[var(--dg-text-muted)] text-center">演示账号</p>
-            <div className="flex gap-2 mt-2">
-              <button onClick={() => { setUsername('admin'); setPassword('123456'); }} className="flex-1 py-2 text-xs rounded-md bg-blue-500/10 text-blue-400 border border-blue-500/20 hover:bg-blue-500/20 transition-colors">
-                管理员
-              </button>
-              <button onClick={() => { setUsername('developer'); setPassword('123456'); }} className="flex-1 py-2 text-xs rounded-md bg-green-500/10 text-green-400 border border-green-500/20 hover:bg-green-500/20 transition-colors">
-                开发者
-              </button>
-            </div>
-          </div>
         </div>
       </div>
     </div>