feat: Respect XDG Base Directory Specification on Linux

[imtuyethan]

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Is your feature request related to a problem? Please describe it

I help created request for this user:

Jan currently doesn't follow the XDG Base Directory Specification on Linux systems. This goes against established conventions and best practices for Linux application development.

Describe the solution

Update Jan to respect the XDG Base Directory Specification by moving the Jan folder to the XDG_DATA_HOME directory (usually ~/.local/share/jan).

Teachability, documentation, adoption, migration strategy

What is the motivation / use case for changing the behavior?

• Improve compatibility with Linux systems
• Follow established conventions, making it easier for users to manage app data
• Align with best practices for Linux application development
• Potentially improve system cleanliness by storing data in the expected location