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>

view/adminhtml/web/css/order-items.css

@@ -0,0 +1,11 @@
+/**
+ * Hide product thumbnails in order items view
+ */
+.order-tables .col-product .product-image {
+    display: none !important;
+}
+
+/* Adjust product info spacing when thumbnail is hidden */
+.order-tables .col-product .product-sku-block {
+    margin-left: 0 !important;
+}