refactor(css): remove animated login gradient

Strip the heavy animated multi-layer radial gradient from the login
page. Removes:
- ani-gradient keyframes and 40 @property declarations
- Light and dark theme gradient rules
- Empty #gradient div from login template
- prefers-reduced-motion override that only served the gradient

The login page now uses the plain app background color consistently.

templates/login.phtml

@@ -63,7 +63,7 @@ $loginComponentConfig = [
     </div>
   </main>
   <?php require __DIR__ . '/partials/app-footer.phtml'; ?>
-  <div id="gradient"></div>
+
   <script type="application/json" id="page-config-login-components"><?php gridJsonForJs($loginComponentConfig); ?></script>
   <?php require __DIR__ . '/partials/app-flash.phtml'; ?>
   <script type="module" src="<?php e(assetVersion('js/app-login-init.js')); ?>"></script>