feat(group): add Validate for cross-field group validation

[ChrisJr404]

Refs #386.

huh lets you validate individual fields, but there was no way to validate a group as a whole. This adds Group.Validate(func() error) that runs when the form tries to advance past the group. If it returns an error the form stays put and shows the message, just like a field error does.

It's meant for checks that span more than one field, like confirming two inputs match or that a date range makes sense. The func closes over your bound values, so you can look at whatever you need:

huh.NewGroup(
    huh.NewInput().Value(&password).Title("Password"),
    huh.NewInput().Value(&confirm).Title("Confirm"),
).Validate(func() error {
    if password != confirm {
        return errors.New("passwords do not match")
    }
    return nil
})

Groups without a Validate behave exactly as before. The error clears when you go back a group, and it re-runs every time you try to move forward, so fixing the input and advancing again just works.

This only covers the synchronous case from the issue. I left the async/spinner idea out since that's a bigger design question, happy to look at it separately if you want it.