auth: build verification link from BASE_URL env var

- Build the verification URL as /api/auth/verify-email?token=xxx&callbackURL=/login
- Pass token from Better Auth callback instead of its built-in url
- Add callbackURL=/login so user lands on login after verifying

src/lib/auth.ts

@@ -13,10 +13,10 @@ export const auth = betterAuth({
 	}),
 	emailVerification: {
 		sendOnSignUp: true,
-		sendVerificationEmail: async ({ user, url }) => {
+		sendVerificationEmail: async ({ user, token }) => {
 			void sendVerificationEmail({
 				email: user.email,
-				verificationUrl: url
+				token
 			});
 		}
 	},