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

components/oauth.tsx

@@ -20,7 +20,7 @@ export const OAuth = ({ title }: OAuthProps) => {
       const result = await googleOAuth(startOAuthFlow);
 
       if (result?.code === "session_exists" || result?.code === "success") {
-        router.replace("/(root)/(tabs)/home");
+        router.replace("/");
       }
     } catch (err: any) {
       console.error("OAuth error", err);