design: More clear chat background and rounded popup menu

lib/pages/chat/chat_view.dart

@@ -221,10 +221,10 @@ class ChatView extends StatelessWidget {
                             gradient: LinearGradient(
                               begin: Alignment.topCenter,
                               colors: [
-                                colorScheme.primaryContainer.withAlpha(32),
-                                colorScheme.secondaryContainer.withAlpha(32),
-                                colorScheme.tertiaryContainer.withAlpha(32),
-                                colorScheme.primaryContainer.withAlpha(32),
+                                colorScheme.primaryContainer.withAlpha(64),
+                                colorScheme.secondaryContainer.withAlpha(64),
+                                colorScheme.tertiaryContainer.withAlpha(64),
+                                colorScheme.primaryContainer.withAlpha(64),
                               ],
                             ),
                           ),

lib/widgets/m2_popup_menu_button.dart

@@ -1,5 +1,7 @@
 import 'package:flutter/material.dart';
 
+import 'package:fluffychat/config/app_config.dart';
+
 class M2PopupMenuButton<T> extends StatelessWidget {
   final List<PopupMenuEntry<T>> Function(BuildContext) itemBuilder;
   final T? initialValue;
@@ -28,6 +30,9 @@ class M2PopupMenuButton<T> extends StatelessWidget {
         useMaterial3: false,
         popupMenuTheme: PopupMenuThemeData(
           color: theme.colorScheme.surface,
+          shape: RoundedRectangleBorder(
+            borderRadius: BorderRadius.circular(AppConfig.borderRadius),
+          ),
           elevation: theme.appBarTheme.scrolledUnderElevation,
           textStyle: theme.textTheme.bodyText1,
         ),