idea: Add "Delete All Threads" functionality

[imtuyethan]

Problem Statement

Currently there is no way to bulk delete all conversation threads in Jan, requiring users to delete threads one by one which can be time-consuming.

Feature Idea

Add a "Delete All Threads" button in the advanced settings with:

• Modal confirmation that says "Delete all chat history?"
• Clear warning that this action cannot be undone and will remove all conversation threads in {file_path_here}
• Delete (red) and Cancel buttons for confirmation
• Consider showing the total number of threads that will be deleted

This would give users an easy way to clear their chat history while ensuring they don't accidentally delete important conversations through the confirmation modal.