feat: implement xdg-desktop-portal in Jan for better Linux integration

[louis-jan]

Summary

Implement support for xdg-desktop-portal to improve Jan's integration with Linux desktop environments and enable better sandboxing support.

Background

xdg-desktop-portal is a standardized D-Bus API that provides desktop integration features in a secure, sandboxed manner. It's the modern standard for Linux applications, especially those distributed via Flatpak, Snap, or running in sandboxed environments.

Benefits

1. Native File Dialogs: Use the desktop environment's native file picker instead of generic dialogs
2. Better Sandboxing: Enable proper sandboxing support for enhanced security
3. Desktop Integration: Improved integration with GNOME, KDE, and other desktop environments
4. Future-Proof: Align with modern Linux desktop standards
5. Flatpak/Snap Support: Better compatibility with containerized distribution formats

Proposed Implementation

• [ ] Integrate xdg-desktop-portal for file picker operations
• [ ] Use portal APIs for system integration (notifications, etc.)
• [ ] Test with various desktop environments (GNOME, KDE, Xfce)
• [ ] Ensure fallback behavior for systems without portal support
• [ ] Update documentation for Linux users

Implementation Considerations

• Tauri may already have some portal support - need to verify current capabilities
• Should maintain backwards compatibility with non-portal systems
• Consider runtime detection of portal availability

Test Plan

• [ ] Test on GNOME with portal support
• [ ] Test on KDE with portal support
• [ ] Test on systems without portal (fallback behavior)
• [ ] Test in Flatpak environment
• [ ] Verify file picker dialogs use native DE styling

References

• xdg-desktop-portal documentation
• Portal API Reference

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