feat: Respect XDG Base Directory Specification on Linux

[imtuyethan]

• I have searched the existing issues

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

[imtuyethan]

After internal discussion, we've decided to keep Jan's current directory structure since:

1. Jan is designed to self-manage its data folder including Cortex data
2. The current implementation allows Jan to properly handle its data isolation
3. This approach enables Jan to maintain proper separation between Jan and Cortex components
4. The current Jan app location in ~/.config/Jan actually follows common app formatting conventions. Since Jan manages its own data folder structure and this is working as intended.