Add in-app autostart management support for Snap builds

[ariyadey]

Currently, in-app autostart management is only supported for deb and rpm package builds. Add support to enable, disable, and check autostart status from within the app (system tray and preferences) when bundled as a Snap package.

Expected Changes:

• Detect Snap runtime at runtime (if not already handled).
• Implement correct Snap-compliant way to manage autostart (verify support for autostart .desktop files under Snap confinement).
• Enable and test autostart tray menu item for Snap builds.
• Update user and developer documentation for Snap specifics and limitations.
• Test thoroughly in a Snap environment.

References:

• Tauri Snap documentation
• Snapcraft forums on desktop autostart