style(ui): refine login card shape and divider spacing

- Remove top border-radius from AuthCard (flat top edge, rounded bottom)
- Add overflow-hidden so rainbow gradient bar stays within card bounds
- Remove rounded-t-2xl from gradient bar (no longer needed)
- Increase AuthDivider vertical margin from my-5 to my-8
- Update test selector to match new rounded-b-2xl class

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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

@@ -491,8 +491,8 @@ describe("LoginPage", (): void => {
         expect(screen.getByLabelText(/email/i)).toBeInTheDocument();
       });
 
-      // AuthCard uses rounded-2xl and p-6 sm:p-10
-      const card = container.querySelector(".rounded-2xl");
+      // AuthCard uses rounded-b-2xl and p-6 sm:p-10
+      const card = container.querySelector(".rounded-b-2xl");
       expect(card).toHaveClass("p-6", "sm:p-10");
     });