Rename module to AdminOrderColumns and add thumbnail removal

Major changes:
- Renamed module from OrderItemsCustom to AdminOrderColumns for clarity
- Added CSS to hide product thumbnails in order items view
- Updated all namespaces from Shopkeeper\OrderItemsCustom to Shopkeeper\AdminOrderColumns
- Updated composer package name to shopkeeper/module-admin-order-columns
- Bumped version to 1.1.0
- Added layout XML and CSS files for thumbnail removal
- Updated all documentation and installation instructions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

registration.php

@@ -3,6 +3,6 @@ use Magento\Framework\Component\ComponentRegistrar;
 
 ComponentRegistrar::register(
     ComponentRegistrar::MODULE,
-    'Shopkeeper_OrderItemsCustom',
+    'Shopkeeper_AdminOrderColumns',
     __DIR__
 );