feat: add unit mapping functionality

- Added new API path for unit mappings in `api_paths.dart`.
- Implemented `upsertUnitMapping` method in `ImportRepository` to handle unit mapping creation.
- Updated `ReceiptImportTab` to learn and save unit mappings during receipt import.
- Created DTO for unit mapping with validation in `create-unit-mapping.dto.ts`.
- Added SQL migration for `UnitMapping` table creation with necessary constraints.

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

@@ -5,6 +5,7 @@ import 'dart:typed_data';
 import 'package:http/http.dart' as http;
 import 'dart:developer' as developer;
 
+import '../../../core/api/api_paths.dart';
 import '../../../core/api/api_exception.dart';
 import '../domain/quick_import_result.dart';
 
@@ -215,6 +216,45 @@ class ImportRepository {
     }
   }
 
+  Future<void> upsertUnitMapping({
+    required int productId,
+    required String originalUnit,
+    required String preferredUnit,
+    String? token,
+  }) async {
+    final normalizedOriginalUnit = originalUnit.trim().toLowerCase();
+    final normalizedPreferredUnit = preferredUnit.trim().toLowerCase();
+
+    if (normalizedOriginalUnit.isEmpty || normalizedPreferredUnit.isEmpty) {
+      return;
+    }
+    if (normalizedOriginalUnit == normalizedPreferredUnit) {
+      return;
+    }
+
+    final uri = Uri.parse('$_baseUrl${ReceiptImportApiPaths.unitMappings}');
+    final response = await _client.post(
+      uri,
+      headers: {
+        'Content-Type': 'application/json',
+        if (token != null) 'Authorization': 'Bearer $token',
+      },
+      body: jsonEncode({
+        'productId': productId,
+        'originalUnit': normalizedOriginalUnit,
+        'preferredUnit': normalizedPreferredUnit,
+      }),
+    );
+
+    if (response.statusCode < 200 || response.statusCode >= 300) {
+      throw ApiException(
+        type: _mapStatusCodeToErrorType(response.statusCode),
+        message: 'Kunde inte spara enhetsmappning: ${response.body}',
+        statusCode: response.statusCode,
+      );
+    }
+  }
+
   /// Helper method to map HTTP status codes to [ApiErrorType].
   ApiErrorType _mapStatusCodeToErrorType(int statusCode) {
     if (statusCode == 401) return ApiErrorType.unauthorized;