feat: Add zone management backend + drawer z-index fix + SettingsCard action prop

## 1. Fixed Drawer Z-Index ✅
- Increased drawer z-index from 50 to 60
- Now appears above bottom navigation (z-50)
- Fixes mobile drawer visibility issue

## 2. Zone Management Backend ✅
Added full CRUD for shipping zones:
- POST /settings/shipping/zones - Create zone
- PUT /settings/shipping/zones/{id} - Update zone
- DELETE /settings/shipping/zones/{id} - Delete zone
- GET /settings/shipping/locations - Get countries/states/continents

Features:
- Create zones with name and regions
- Update zone name and regions
- Delete zones
- Region selector with continents, countries, and states
- Proper cache invalidation

## 3. Zone Management Frontend (In Progress) ⏳
- Added state for zone CRUD (showAddZone, editingZone, deletingZone)
- Added mutations (createZone, updateZone, deleteZone)
- Added "Add Zone" button to SettingsCard
- Updated empty state with "Create First Zone" button

## 4. Enhanced SettingsCard Component ✅
- Added optional `action` prop for header buttons
- Flexbox layout for title/description + action
- Used in Shipping zones for "Add Zone" button

## Next Steps:
- Add delete button to each zone
- Create Add/Edit Zone dialog with region selector
- Add delete confirmation dialog
- Then move to Tax rates and Email subjects

admin-spa/src/components/ui/drawer.tsx

@@ -26,7 +26,7 @@ const DrawerOverlay = React.forwardRef<
 >(({ className, ...props }, ref) => (
   <DrawerPrimitive.Overlay
     ref={ref}
-    className={cn("fixed inset-0 z-50 bg-black/80", className)}
+    className={cn("fixed inset-0 z-[60] bg-black/80", className)}
     {...props}
   />
 ))
@@ -41,7 +41,7 @@ const DrawerContent = React.forwardRef<
     <DrawerPrimitive.Content
       ref={ref}
       className={cn(
-        "fixed inset-x-0 bottom-0 z-50 mt-24 flex h-auto flex-col rounded-t-[10px] border bg-background",
+        "fixed inset-x-0 bottom-0 z-[60] mt-24 flex h-auto flex-col rounded-t-[10px] border bg-background",
         className
       )}
       {...props}