Keep Shift+Tab working off an invalid page in the project create wizard

huh v2.0.3 blurs the focused field and then asks for the previous page on Shift+Tab. The form refuses while the page has a validation error and never refocuses the field, so after an invalid name (a taken one, or an empty one) Shift+Tab left the page ignoring every key until Ctrl+C.

Add uiform.BackNav, which watches keys through a tea filter, and uiform.Lenient, which passes a validator while the back key is being handled. The name page uses it, so going back needs no valid answer and Enter still validates going forward. Shared so the repo create wizard can use it too.

Found and fixed by the repo create session.