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)/_layout.tsx

@@ -7,6 +7,11 @@ const RootLayout = () => {
       <Stack.Screen name="find-ride" options={{ headerShown: false }} />
       <Stack.Screen name="confirm-ride" options={{ headerShown: false }} />
       <Stack.Screen name="book-ride" options={{ headerShown: false }} />
+      <Stack.Screen name="role" options={{ headerShown: false }} />
+      <Stack.Screen
+        name="driver-home"
+        options={{ headerShown: false, gestureEnabled: false }}
+      />
     </Stack>
   );
 };