fix(mobile): Mark more strings for translation (#5132)

* fix(mobile): Mark more strings for translation

Moving more strings to the `i18n` JSON file, and also including their
es-US translations.

* Add more translatable strings

mobile/lib/modules/login/ui/login_form.dart

@@ -358,7 +358,7 @@ class LoginForm extends HookConsumerWidget {
             TextButton.icon(
               icon: const Icon(Icons.arrow_back),
               onPressed: () => serverEndpoint.value = null,
-              label: const Text('Back'),
+              label: const Text('login_form_back_button_text').tr(),
             ),
           ],
         ),