feat(m1): 应用骨架、传送门面板与暂存区

- CMake 构建（Qt6 + dtk2widget + LayerShellQt）
- 单实例 / --daemon / --show
- DBlur 双栏面板：传送门 + 暂存区，layer-shell 定位，不抢焦点
- 暂存区模型：文件/文字/图片、去重、JSON 持久化、文件消失清理
- 列表委托：复选框、缩略图、名称、hover 关闭按钮
- 全选 / 反选 / 删除选中
- 应用图标（多尺寸 hicolor）与 desktop 入口
- 方案更新：确认 layer-shell + 遮挡自动隐藏

src/app.h

@@ -0,0 +1,38 @@
+// SPDX-FileCopyrightText: 2026 GXDE OS Team
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#pragma once
+
+#include <QObject>
+
+class PortalPanel;
+class QLocalServer;
+class StagingModel;
+
+/**
+ * Application controller: owns the staging model, the panel and the single
+ * instance machinery.
+ */
+class App : public QObject
+{
+    Q_OBJECT
+
+public:
+    explicit App(QObject *parent = nullptr);
+
+    /**
+     * @param daemon    run in the background, only show the panel when needed
+     * @param forceShow always show the panel on startup
+     * @return false when another instance is already running
+     */
+    bool init(bool daemon, bool forceShow);
+
+private:
+    void ensurePanel();
+    void showPanel();
+
+    StagingModel *m_model = nullptr;
+    PortalPanel *m_panel = nullptr;
+    QLocalServer *m_server = nullptr;
+    bool m_daemon = false;
+};