feat: add HelpText model, service, and controller for dynamic help text management

flutter/lib/features/import/domain/help_text_content.dart

@@ -0,0 +1,27 @@
+class HelpTextContent {
+  final String key;
+  final String scope;
+  final String title;
+  final String content;
+  final DateTime? updatedAt;
+
+  const HelpTextContent({
+    required this.key,
+    required this.scope,
+    required this.title,
+    required this.content,
+    this.updatedAt,
+  });
+
+  factory HelpTextContent.fromJson(Map<String, dynamic> json) {
+    return HelpTextContent(
+      key: (json['key'] as String? ?? '').trim(),
+      scope: (json['scope'] as String? ?? 'default').trim(),
+      title: (json['title'] as String? ?? '').trim(),
+      content: (json['content'] as String? ?? '').trim(),
+      updatedAt: json['updatedAt'] is String
+          ? DateTime.tryParse(json['updatedAt'] as String)
+          : null,
+    );
+  }
+}