feat(chat,todo): 小埃同学对话窗口与待办窗口

Chat:
- DTK2 窗口：左侧会话列表 + 右侧对话区 + 输入框
- DeepSeek / 自定义 API，SSE 流式输出，首字即显示
- 会话持久化到 chat.json，新建/切换/标题自动生成
- 设置对话框：API Key、API 地址、模型、系统提示，含开通指引
- 拖放：左侧空白=新建对话，左侧会话=指定会话，右侧对话区=当前会话
- Ctrl+Enter 发送，错误以消息形式回显

Todo:
- 待办窗口（layer-shell，不抢焦点、不进 dock），复选框/删除/新建
- 待办持久化 todo.json，--todo 打开，独立 desktop 入口

其他:
- desktop 入口改为打开对话窗口，--chat 参数
- 单实例消息支持 show/todo/chat

src/todo/todowindow.h

@@ -0,0 +1,41 @@
+// SPDX-FileCopyrightText: 2026 GXDE OS Team
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#pragma once
+
+#include <DBlurEffectWidget>
+#include <QWidget>
+
+class QLabel;
+class QListView;
+class TodoModel;
+
+/**
+ * Floating todo window (optional keep-above, draggable, hidden from the dock).
+ *
+ * On Wayland it is a layer-shell overlay surface (keyboard interactivity is
+ * none so it never steals focus); new entries are added from the panel or the
+ * "+" button which opens a normal dialog.
+ */
+class TodoWindow : public Dtk::Widget::DBlurEffectWidget
+{
+    Q_OBJECT
+
+public:
+    explicit TodoWindow(TodoModel *model, QWidget *parent = nullptr);
+
+    void popup();
+    void refreshLayout();
+
+protected:
+    void showEvent(QShowEvent *event) override;
+
+private:
+    void setupWindow();
+    void setupUi();
+    void addEntry();
+
+    TodoModel *m_model = nullptr;
+    QListView *m_view = nullptr;
+    QLabel *m_countLabel = nullptr;
+};