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/index.tsx

@@ -1,12 +1,38 @@
 import { useAuth } from "@clerk/clerk-expo";
 import { Redirect } from "expo-router";
+import { useEffect, useState } from "react";
+import { ActivityIndicator, View } from "react-native";
+
+import { fetchAPI } from "@/lib/fetch";
 
 const App = () => {
-  const { isSignedIn } = useAuth();
+  const { isSignedIn, userId } = useAuth();
+  const [role, setRole] = useState<string | null | undefined>(undefined);
 
-  if (isSignedIn) return <Redirect href="/(root)/(tabs)/home" />;
+  useEffect(() => {
+    if (!isSignedIn || !userId) return;
 
-  return <Redirect href="/(auth)/welcome" />;
+    fetchAPI(`/(api)/user?clerkId=${userId}`)
+      .then((res) => setRole(res?.data?.role ?? null))
+      .catch(() => setRole(null));
+  }, [isSignedIn, userId]);
+
+  if (!isSignedIn) return <Redirect href="/(auth)/welcome" />;
+
+  // Still loading the user's role from the database.
+  if (role === undefined) {
+    return (
+      <View className="flex-1 items-center justify-center bg-white">
+        <ActivityIndicator size="large" color="#0286FF" />
+      </View>
+    );
+  }
+
+  if (role === "driver") return <Redirect href="/(root)/driver-home" />;
+  if (role === "rider") return <Redirect href="/(root)/(tabs)/home" />;
+
+  // Signed in but no role chosen yet.
+  return <Redirect href="/(root)/role" />;
 };
 
 export default App;