feat(portal): 接入合成器传送门，暂存区固定/消耗语义

- 新增 PortalClient：org.gxde.Portal 的 BeginPortal / CancelPortal 与 drag 信号
- 传送门区域悬停 150ms 后打开合成器 overview；无窗口/不支持时给出提示
- 拖拽开始时自动弹出面板（120ms 延迟，mime 过滤），拖拽结束自动收起
- 暂存区每行新增固定按钮；默认拖出后消耗，固定后保留；取消拖拽不消耗
- 拒绝自身拖拽落入暂存区，避免重复条目

src/portal/portalclient.h

@@ -0,0 +1,46 @@
+// SPDX-FileCopyrightText: 2026 GXDE OS Team
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#pragma once
+
+#include <QObject>
+
+/**
+ * Client for the compositor side "portal" (org.gxde.Portal).
+ *
+ * Start with \a beginPortal() while a drag hovers the portal zone.  The
+ * compositor opens the window overview in portal mode and forwards the drag
+ * to the window under the cursor, so the target application receives a real
+ * wl_data_device drop at the cursor position.
+ */
+class PortalClient : public QObject
+{
+    Q_OBJECT
+
+public:
+    static PortalClient *instance();
+
+    bool available() const { return m_available; }
+
+    /** @return true when the compositor opened the overview. */
+    bool beginPortal();
+    void cancelPortal();
+
+    /** Filter the drag mime types we want to react to. */
+    static bool mimeInteresting(const QString &mimeTypes);
+
+signals:
+    void dragStarted(const QString &mimeTypes);
+    void dragEnded();
+    void dropped(const QString &uuid, bool accepted);
+
+private slots:
+    void onDragStarted(const QString &mimeTypes);
+    void onDragEnded();
+    void onDropped(const QString &uuid, bool accepted);
+
+private:
+    explicit PortalClient(QObject *parent = nullptr);
+
+    bool m_available = false;
+};