give parents their own way in instead of the student setup form

A parent account has no grade, so the app sent the demo mother to the
"which grade are you in?" form and she never reached the parents screen.
Now an account with a name, no grade and a parent-side family link opens
straight on the parents screen (with a logout button, since it has no tab
bar), and the setup form offers "I'm a parent, not a student" for new ones.
The header's back button only shows when there is somewhere to go back to.
Found on the live site with the demo accounts.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/components/ui.tsx

@@ -33,7 +33,7 @@ export function ScreenHeader({
       <div className="pointer-events-none absolute -left-12 -top-12 h-40 w-40 rounded-full bg-white/10" />
       <div className="pointer-events-none absolute -right-10 top-10 h-28 w-28 rounded-full bg-white/10" />
       <div className="relative z-10 pt-6">
-        {back && (
+        {back && (typeof back === 'function' || nav.canGoBack) && (
           <IconButton label="بازگشت" onClick={onBack} className="mb-3 bg-white/15">
             <ChevronRight size={18} strokeWidth={2.5} />
           </IconButton>