feat: Jan provides UI option to adjust chat text size for better readability

[eckartal]

Problem
The fixed chat text size in Jan's UI is difficult to read, especially on larger screens, which can strain users' eyes and make the chat interface less accessible.

Success Criteria
Add UI settings option that allows users to adjust the text size of the chat interface. This feature should provide a range of sizes to accommodate different screen sizes and user preferences - improve readability and accessibility.

Additional context
Detailed feedback: https://www.reddit.com/r/LocalLLaMA/comments/1cclmjk/comment/l1byjva/?context=3