fix(loyalty): use confirm modal for category deletion

Replace browser confirm() dialog with the shared confirm_modal
macro for category deletion. Matches the existing program delete
pattern. Shows warning about impact on existing transactions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/modules/loyalty/locales/en.json

@@ -487,6 +487,7 @@
             "transaction_categories": "Transaction Categories",
             "select_store": "Select a store...",
             "no_categories": "No categories configured. Click Add to create one.",
+            "delete_category_message": "Are you sure you want to delete this category? Existing transactions will keep the category reference but it will no longer be available for new transactions.",
             "admin_policy_settings": "Admin Policy Settings",
             "staff_pin_policy": "Staff PIN Policy",
             "self_enrollment": "Self Enrollment",