refactor(01-tech-debt): extract AllegroTokenManager and StringHelper

Phase 1 complete (2/2 plans):

- Plan 01-01: Extract AllegroTokenManager — OAuth token logic
  centralized from 4 classes into dedicated manager class

- Plan 01-02: Extract StringHelper — nullableString/normalizeDateTime/
  normalizeColorHex extracted from 15+ classes into App\Core\Support\StringHelper;
  removed 19 duplicate private methods

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Modules/Settings/InpostIntegrationRepository.php

@@ -3,6 +3,7 @@ declare(strict_types=1);
 
 namespace App\Modules\Settings;
 
+use App\Core\Support\StringHelper;
 use PDO;
 use RuntimeException;
 use Throwable;
@@ -98,15 +99,15 @@ final class InpostIntegrationRepository
             WHERE id = 1'
         );
         $statement->execute([
-            'api_token_encrypted' => $this->nullableString((string) $nextEncrypted),
-            'organization_id' => $this->nullableString(trim((string) ($payload['organization_id'] ?? ''))),
+            'api_token_encrypted' => StringHelper::nullableString((string) $nextEncrypted),
+            'organization_id' => StringHelper::nullableString(trim((string) ($payload['organization_id'] ?? ''))),
             'environment' => in_array($payload['environment'] ?? '', ['sandbox', 'production'], true)
                 ? $payload['environment']
                 : 'sandbox',
             'default_dispatch_method' => in_array($payload['default_dispatch_method'] ?? '', ['pop', 'parcel_locker', 'courier'], true)
                 ? $payload['default_dispatch_method']
                 : 'pop',
-            'default_dispatch_point' => $this->nullableString(trim((string) ($payload['default_dispatch_point'] ?? ''))),
+            'default_dispatch_point' => StringHelper::nullableString(trim((string) ($payload['default_dispatch_point'] ?? ''))),
             'default_insurance' => ($payload['default_insurance'] ?? '') !== ''
                 ? (float) $payload['default_insurance']
                 : null,
@@ -215,9 +216,4 @@ final class InpostIntegrationRepository
         );
     }
 
-    private function nullableString(string $value): ?string
-    {
-        $trimmed = trim($value);
-        return $trimmed === '' ? null : $trimmed;
-    }
 }