fix(auth): restore BetterAuth OIDC flow across api/web/compose

apps/web/src/app/(auth)/login/page.test.tsx

@@ -276,7 +276,7 @@ describe("LoginPage", (): void => {
 
     expect(mockOAuth2).toHaveBeenCalledWith({
       providerId: "authentik",
-      callbackURL: "/",
+      callbackURL: "http://localhost:3000/",
     });
   });
 

apps/web/src/app/(auth)/login/page.tsx

@@ -113,7 +113,9 @@ function LoginPageContent(): ReactElement {
   const handleOAuthLogin = useCallback((providerId: string): void => {
     setOauthLoading(providerId);
     setError(null);
-    signIn.oauth2({ providerId, callbackURL: "/" }).catch((err: unknown) => {
+    const callbackURL =
+      typeof window !== "undefined" ? new URL("/", window.location.origin).toString() : "/";
+    signIn.oauth2({ providerId, callbackURL }).catch((err: unknown) => {
       const message = err instanceof Error ? err.message : String(err);
       console.error(`[Auth] OAuth sign-in initiation failed for ${providerId}:`, message);
       setError("Unable to connect to the sign-in provider. Please try again in a moment.");