FEATURE REQUEST: Allow manually setting default shell per host

[neatchee]

USE CASE:

UI/UX preference for minimalism means I would like to be able to launch my desired shell for a given host without navigating to the specific entry for that shell. For example, on Windows, the Local Machine always uses CMD as the default shell, which means that I need to explicitly select "pwsh" to use Powershell 7 every time

DESIGN

There are two possible options:

1. Simpler: When editing a host (including Local Machine), populate a dropdown for "Default Shell" with known available shells

2. More Consistent with User Expectations/Learned Behavior: When editing a shell environment, add a checkbox for "[make|set as] default for this host"

NOTES

If the selected shell is removed or fails to launch, xpipe should probably fall back to the baked-in default