simplified code across the app and consolidated duplicated helpers

lib/push/push_target.dart

@@ -1,3 +1,16 @@
+import 'dart:convert';
+
+/// Decodes the JSON payload of a locally rendered notification; null when
+/// absent or malformed.
+Map<String, dynamic>? decodeNotificationPayload(String? payload) {
+  if (payload == null || payload.isEmpty) return null;
+  try {
+    return jsonDecode(payload) as Map<String, dynamic>;
+  } on Object {
+    return null;
+  }
+}
+
 /// Where a tapped notification leads.
 sealed class PushTarget {
   const PushTarget();