feat(ds-flow): Phase 4.1 — 清 adaptive_menu 唯一裸 Material 色

adaptive_menu.dart shadowColor: Colors.black26(冷黑)→ PangolinColors.sand950
withValues(0.24)(暖近黑,对齐 §5 暖阴影铁律)。UI 层(widgets/screens/shell)
自此零裸 Material 色。flutter analyze 通过。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/lib/widgets/adaptive_menu.dart

@@ -167,7 +167,7 @@ class _MenuCard extends StatelessWidget {
       child: Material(
         color: c.surface,
         elevation: 6,
-        shadowColor: Colors.black26,
+        shadowColor: PangolinColors.sand950.withValues(alpha: 0.24), // 暖近黑阴影(§5;非冷黑 Colors.black26)
         clipBehavior: Clip.antiAlias,
         shape: RoundedRectangleBorder(
           borderRadius: BorderRadius.circular(PangolinRadius.md),