fix: hide mock credentials

app/login/page.tsx

@@ -62,7 +62,7 @@ export default function LoginPage() {
                 value={username}
                 onChange={(e) => setUsername(e.target.value)}
                 className="w-full px-5 py-3.5 bg-surface-container-high/50 border border-outline-variant/50 rounded-xl focus:ring-2 focus:ring-primary focus:border-transparent outline-none transition-all placeholder:text-outline text-on-surface shadow-inner"
-                placeholder="请输入学号/工号 (stu 或 teacher)"
+                placeholder="请输入学号/工号"
                 required
               />
             </div>
@@ -74,7 +74,7 @@ export default function LoginPage() {
                 value={password}
                 onChange={(e) => setPassword(e.target.value)}
                 className="w-full px-5 py-3.5 bg-surface-container-high/50 border border-outline-variant/50 rounded-xl focus:ring-2 focus:ring-primary focus:border-transparent outline-none transition-all placeholder:text-outline text-on-surface shadow-inner"
-                placeholder="请输入密码 (passwd)"
+                placeholder="请输入密码"
                 required
               />
             </div>