feat(auth): 登录页改为 ymb 品牌并支持后台配置背景图

lib/pages/auth/auth_form_widgets.dart

@@ -300,7 +300,7 @@ class _AuthBrand extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final copy = _brandCopy(context, centered: !compact);
+    final copy = _brandCopy(centered: !compact);
     if (compact) {
       return Row(
         key: const Key('auth-brand'),
@@ -324,32 +324,14 @@ class _AuthBrand extends StatelessWidget {
     radius: compact ? _compactBrandMarkRadius : _brandMarkRadius,
   );
 
-  Widget _brandCopy(BuildContext context, {required bool centered}) => Column(
-    crossAxisAlignment: centered
-        ? CrossAxisAlignment.center
-        : CrossAxisAlignment.start,
-    children: [
-      Text(
-        AppLocalizations.of(context)!.appName,
-        textAlign: centered ? TextAlign.center : TextAlign.start,
-        style: const TextStyle(
-          color: AppColors.onSurface,
-          fontSize: 24,
-          fontWeight: FontWeight.w700,
-          height: 1.1,
-        ),
-      ),
-      const SizedBox(height: 6),
-      Text(
-        tagline,
-        textAlign: centered ? TextAlign.center : TextAlign.start,
-        style: const TextStyle(
-          color: AppColors.muted,
-          fontSize: 13,
-          height: 1.35,
-        ),
-      ),
-    ],
+  Widget _brandCopy({required bool centered}) => Text(
+    tagline,
+    textAlign: centered ? TextAlign.center : TextAlign.start,
+    style: const TextStyle(
+      color: AppColors.muted,
+      fontSize: 13,
+      height: 1.35,
+    ),
   );
 }