feat(auth): 登录页改为 ymb 品牌并支持后台配置背景图

lib/core/widgets/remote_image.dart

@@ -35,6 +35,7 @@ class RemoteImage extends StatelessWidget {
     this.fallbackIconKey,
     this.fallbackAspectRatio,
     this.fallbackColor = AppColors.raised,
+    this.fallbackWidget,
     this.decodeWidth,
     this.decodeHeight,
   });
@@ -47,6 +48,9 @@ class RemoteImage extends StatelessWidget {
   final Key? fallbackIconKey;
   final double? fallbackAspectRatio;
   final Color fallbackColor;
+
+  /// 自定义兜底：地址为空、加载中、加载失败时都显示它（优先于 [fallbackColor]）。
+  final Widget? fallbackWidget;
   final int? decodeWidth;
   final int? decodeHeight;
 
@@ -77,9 +81,9 @@ class RemoteImage extends StatelessWidget {
         // cover/contain 本身会保持宽高比，此时若同时强制 cacheWidth 和
         // cacheHeight，解码会把竖版图强行拉成横版像素。只在显式指定了
         // decodeHeight、或 fit 不保持宽高比时才限定解码高度。
-        final keepsAspectRatio =
-            fit == BoxFit.cover || fit == BoxFit.contain;
-        final cacheHeight = decodeHeight ??
+        final keepsAspectRatio = fit == BoxFit.cover || fit == BoxFit.contain;
+        final cacheHeight =
+            decodeHeight ??
             (keepsAspectRatio || inferredHeight == null
                 ? null
                 : resolveImageDecodePixels(inferredHeight, pixelRatio));
@@ -106,6 +110,8 @@ class RemoteImage extends StatelessWidget {
   }
 
   Widget _fallback({required bool brand}) {
+    final custom = fallbackWidget;
+    if (custom != null) return custom;
     final mark = brand
         ? LayoutBuilder(
             builder: (context, constraints) {