Add in-app autostart management support for Flatpak builds

[ariyadey]

Currently, in-app autostart management is only available when building Capturist as deb or rpm packages. Add support to properly manage and toggle the "autostart" feature from within the app (system tray and preferences) when the app is bundled as a Flatpak.

Expected Changes:

• Detect Flatpak runtime at runtime (if not already handled).
• Implement reading and updating autostart state for Flatpak (possibly XDG autostart, .desktop file in user directories, or Flatpak-specific APIs if available).
• Ensure GUI menu for toggling autostart is enabled and works for Flatpak builds.
• Update documentation to reflect Flatpak specifics and limitations.
• Thoroughly test on Flatpak environment.

References:

• Tauri Flatpak documentation
• GNOME/Flatpak autostart discussion