feat: Update Orders to follow CRUD pattern SOP

Following PROJECT_SOP.md section 5.7 CRUD Module Pattern:

**Backend (NavigationRegistry.php):**
- Added Orders submenu: All orders | New
- Prepared for future tabs (Drafts, Recurring)

**Frontend (Orders/index.tsx):**
- Removed 'New order' button from toolbar
- Kept bulk actions (Delete) in toolbar
- Filters remain in toolbar

**Result:**
- Orders now consistent with Products pattern
- Follows industry standard (Shopify, WooCommerce)
- Submenu for main actions, toolbar for filters/bulk actions

**Next:**
- Implement variable product handling in OrderForm

admin-spa/src/routes/Orders/index.tsx

@@ -245,12 +245,6 @@ export default function Orders() {
       <div className="hidden md:block rounded-lg border border-border p-4 bg-card">
         <div className="flex flex-col lg:flex-row lg:justify-between lg:items-center gap-3">
           <div className="flex gap-3">
-            <button 
-              className="border rounded-md px-3 py-2 text-sm bg-black text-white disabled:opacity-50" 
-              onClick={() => nav('/orders/new')}
-            >
-              {__('New order')}
-            </button>
             {selectedIds.length > 0 && (
               <button
                 className="border rounded-md px-3 py-2 text-sm bg-red-600 text-white hover:bg-red-700 disabled:opacity-50 inline-flex items-center gap-2"