Integrations DI refactor, remove Sellasist/Baselinker, fix product-edit encoding (0.263)

- New: Domain\Integrations\IntegrationsRepository + admin\Controllers\IntegrationsController (DI)
- Cleanup: removed all Sellasist and Baselinker integrations from entire project
- Fix: product-edit.php Polish characters (UTF-8/CP1250 double-encoding)
- Update: factory\Integrations as facade (Apilo + ShopPRO only)
- Tests: 212 tests, 577 assertions

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

admin/templates/site/main-layout.php

@@ -134,21 +134,11 @@
             Integracje
           </div>
           <ul>
-            <li>
-              <a href="/admin/integrations/sellasist_settings/">
-                <i class="fa fa-cogs" aria-hidden="true"></i>sellasist.pl
-              </a>
-            </li>
             <li>
               <a href="/admin/integrations/apilo_settings/">
                 <i class="fa fa-cogs" aria-hidden="true"></i>apilo.com
               </a>
             </li>
-            <li>
-              <a href="/admin/integrations/baselinker_settings/">
-                <i class="fa fa-cogs" aria-hidden="true"></i>baselinker.com
-              </a>
-            </li>
             <li>
               <a href="/admin/integrations/shoppro_settings/">
                 <i class="fa fa-cogs" aria-hidden="true"></i>shopPRO