refactor: 设置面板改为右侧抽屉，修复布局跳动问题

- ConfigPanel 重写为 Drawer：右侧 320px 滑入面板 + 半透明遮罩
- 设置按钮始终可见，不依赖连接状态
- 抽屉不推挤主布局，点击遮罩或✕关闭
- 设置项分组显示，带功能描述文案

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/App.tsx

@@ -54,15 +54,13 @@ function App() {
           <span className={`badge badge--${connectionStatus}`}>
             {isConnected ? "已连接" : connectionStatus === "connecting" ? "连接中..." : "未连接"}
           </span>
-          {isConnected && (
-            <button
-              className="btn-icon"
-              onClick={() => setShowConfig((v) => !v)}
-              title="设置"
-            >
-              ⚙️
-            </button>
-          )}
+          <button
+            className="btn-icon"
+            onClick={() => setShowConfig((v) => !v)}
+            title="设置"
+          >
+            ⚙️
+          </button>
         </div>
       </header>