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/(root)/driver-home.tsx

@@ -0,0 +1,41 @@
+import { useClerk, useUser } from "@clerk/clerk-expo";
+import { Image, Text, View } from "react-native";
+import { SafeAreaView } from "react-native-safe-area-context";
+
+import { CustomButton } from "@/components/custom-button";
+import { images } from "@/constants";
+
+// Placeholder driver home. The driver experience (going online, accepting
+// rides) is not built yet — drivers are registered here and managed in the
+// database for now.
+const DriverHome = () => {
+  const { user } = useUser();
+  const { signOut } = useClerk();
+
+  return (
+    <SafeAreaView className="flex-1 bg-white justify-center items-center px-7">
+      <Image
+        source={images.check}
+        alt="Registered"
+        className="w-[110px] h-[110px] mb-5"
+      />
+
+      <Text className="text-2xl font-JakartaBold text-center">
+        You&apos;re registered as a driver, {user?.firstName || "there"}!
+      </Text>
+
+      <Text className="text-base text-general-200 font-Jakarta text-center mt-3">
+        Driver mode is coming soon. We&apos;ll contact you at{" "}
+        {user?.emailAddresses[0]?.emailAddress} once your account is activated.
+      </Text>
+
+      <CustomButton
+        title="Sign Out"
+        onPress={() => signOut()}
+        className="mt-10"
+      />
+    </SafeAreaView>
+  );
+};
+
+export default DriverHome;