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

flutter/lib/features/import/data/import_repository.dart

@@ -7,6 +7,7 @@ import 'dart:developer' as developer;
 
 import '../../../core/api/api_paths.dart';
 import '../../../core/api/api_exception.dart';
+import '../domain/help_text_content.dart';
 import '../domain/quick_import_result.dart';
 
 /// Handles communication with the quick-import API endpoint.
@@ -25,6 +26,38 @@ class ImportRepository {
           defaultValue: '/api',
         );
 
+  Future<HelpTextContent> fetchHelpTextByKey(
+    String key, {
+    String? token,
+  }) async {
+    final uri = Uri.parse('$_baseUrl${HelpTextApiPaths.byKey(key)}');
+    final response = await _client.get(
+      uri,
+      headers: {
+        'Content-Type': 'application/json',
+        if (token != null) 'Authorization': 'Bearer $token',
+      },
+    );
+
+    if (response.statusCode < 200 || response.statusCode >= 300) {
+      throw ApiException(
+        type: _mapStatusCodeToErrorType(response.statusCode),
+        message: 'Kunde inte hämta hjälptext: ${response.body}',
+        statusCode: response.statusCode,
+      );
+    }
+
+    final parsed = _parseResponse(response);
+    if (parsed is! Map<String, dynamic>) {
+      throw ApiException(
+        type: ApiErrorType.unknown,
+        message: 'Felaktigt svar vid hämtning av hjälptext.',
+      );
+    }
+
+    return HelpTextContent.fromJson(parsed);
+  }
+
   /// Upload a receipt file for parsing (Fas 6b).
   /// Returns a list of parsed receipt items.
   Future<List<ParsedReceiptItem>> importReceiptFile({