Add rider/driver role selection after sign-up

- users.role column (+ migration in seed script)
- /(api)/user: GET role by clerkId, PATCH to set role
- Role selection screen; drivers get placeholder driver-home
- Root index routes by role; hardened location handling on home

app/(auth)/sign-up.tsx

@@ -236,7 +236,7 @@ const SignUp = () => {
 
             <CustomButton
               title="Browse Home"
-              onPress={() => router.push("/(root)/(tabs)/home")}
+              onPress={() => router.push("/")}
               className="mt-5"
             />
           </View>