Set default for output directories in TUI

[nightlark]

The TUI isn't happy if you forget to select an output directory for things like the context file; from a UX standpoint, setting a safe default (the current working directory) could be good. More extensive would be making a custom file selection control that combines the folder and file selection in some way (perhaps modeled after the Windows file selection dialog where it shows the name of the selected file/folder, with the ability to type in a custom name).