refactor: 将 DApplication 替换为 QGuiApplication + DGuiApplicationHelper
- Replace DApplication (dtkwidget) with QGuiApplication + DGuiApplicationHelper (dtkgui)
- Use DGuiApplicationHelper::setSingleInstance() instead of DApplication::setSingleInstance()
- Use DGuiApplicationHelper::loadTranslator() instead of DApplication::loadTranslator()
- Remove DAboutDialog-related settings (productName, productIcon, etc.) as they are only needed by DAboutDialog
- Update CMakeLists.txt to link against Dtk6::Gui instead of Dtk6::Widget
- Remove Qt6::Widgets dependency from the music player target
- 使用 QGuiApplication + DGuiApplicationHelper (dtkgui) 替代 DApplication (dtkwidget)
- 使用 DGuiApplicationHelper::setSingleInstance() 替代 DApplication::setSingleInstance()
- 使用 DGuiApplicationHelper::loadTranslator() 替代 DApplication::loadTranslator()
- 移除 DAboutDialog 相关设置(productName、productIcon 等),这些仅 DAboutDialog 使用
- 更新 CMakeLists.txt,链接 Dtk6::Gui 替代 Dtk6::Widget
- 移除 music player 目标的 Qt6::Widgets 依赖