completed group batch feature

lib/widgets/app_snack_bar.dart

@@ -0,0 +1,77 @@
+import 'package:flutter/material.dart';
+
+/// Centralized helper for showing SnackBars across the app.
+///
+/// This guarantees that every SnackBar has a fixed width of 400 and a
+/// consistent floating behaviour, so the project-wide limit only has to
+/// be set in one place.
+class AppSnackBar {
+  AppSnackBar._();
+
+  /// The default width applied to every floating SnackBar in the app.
+  static const double width = 400;
+
+  /// Shows a SnackBar with the project-wide width of 400.
+  ///
+  /// By default a [Text] widget with [message] is used as the content.
+  /// Pass [customContent] to override the default content widget (e.g. for
+  /// custom layouts that include icons, multiple lines, etc.).
+  static void show(
+    BuildContext context,
+    String message, {
+    Widget? customContent,
+    Color? backgroundColor,
+    Color? textColor,
+    SnackBarBehavior behavior = SnackBarBehavior.floating,
+    SnackBarAction? action,
+    Duration duration = const Duration(seconds: 4),
+    ShapeBorder? shape,
+    EdgeInsetsGeometry? margin,
+    EdgeInsetsGeometry? padding,
+  }) {
+    final messenger = ScaffoldMessenger.maybeOf(context);
+    if (messenger == null) return;
+
+    final Widget content = customContent ??
+        (textColor != null
+            ? Text(message, style: TextStyle(color: textColor))
+            : Text(message));
+
+    messenger.showSnackBar(
+      SnackBar(
+        content: content,
+        backgroundColor: backgroundColor,
+        behavior: behavior,
+        action: action,
+        duration: duration,
+        shape: shape,
+        margin: margin,
+        padding: padding,
+        width: width,
+      ),
+    );
+  }
+
+  /// Convenience helper for error messages.
+  static void showError(BuildContext context, String message) {
+    show(
+      context,
+      message,
+      backgroundColor: Colors.deepOrange,
+    );
+  }
+
+  /// Convenience helper for success messages.
+  static void showSuccess(BuildContext context, String message) {
+    show(
+      context,
+      message,
+      backgroundColor: Colors.green.shade700,
+    );
+  }
+
+  /// Convenience helper for informational messages.
+  static void showInfo(BuildContext context, String message) {
+    show(context, message);
+  }
+}