feat: add AppImage and Flatpak packaging support

- Add AppImage build using linuxdeploy
- Add Flatpak build using flatpak-builder
- Create AppRun script for .NET runtime sharing
- Create Flatpak manifest and desktop file
- Integrate both formats into release workflow
- Upload .AppImage and .flatpak as release artifacts

This provides comprehensive Linux packaging support:
- DEB: For Debian/Ubuntu systems
- Linglong: For Deepin/UOS systems
- AppImage: Universal Linux format (download and run)
- Flatpak: Universal Linux format (sandboxed)

All formats share the same .NET runtime for efficiency.

packaging/linux/Flatpak/com.lanmountain.desktop.desktop

@@ -0,0 +1,10 @@
+[Desktop Entry]
+Type=Application
+Version=1.0
+Name=LanMountainDesktop
+Comment=LanMountainDesktop desktop shell
+Exec=LanMountainDesktop.Launcher %U
+Icon=com.lanmountain.desktop
+Terminal=false
+Categories=Utility;Education;
+StartupWMClass=LanMountainDesktop