feat(checkout): fix searchable select API search and add billing destination

Fixes:
1. SearchableSelect now supports onSearch prop for API-based search
   - Added onSearch and isSearching props
   - shouldFilter disabled when onSearch provided
2. DynamicCheckoutField connects handleApiSearch to SearchableSelect
3. RAJAONGKIR_INTEGRATION.md adds both billing and shipping destination_id

This enables the destination search field to actually call the API
when user types, instead of just filtering local (empty) options.

RAJAONGKIR_INTEGRATION.md

@@ -88,7 +88,7 @@ function woonoow_rajaongkir_search_destinations($request) {
 }
 
 // ============================================================
-// 2. Add destination field to checkout fields
+// 2. Add destination field to checkout fields (both billing and shipping)
 //    Always add for Indonesia zone (no premature country check)
 // ============================================================
 add_filter('woocommerce_checkout_fields', function($fields) {
@@ -103,12 +103,11 @@ add_filter('woocommerce_checkout_fields', function($fields) {
         return $fields;
     }
     
-    // Add searchable destination field
-    // The frontend will show/hide based on selected country
-    $fields['shipping']['shipping_destination_id'] = [
+    // Destination field definition (reused for billing and shipping)
+    $destination_field = [
         'type'            => 'searchable_select',
         'label'           => __('Destination (Province, City, Subdistrict)', 'woonoow'),
-        'required'        => false, // Not required initially, SPA will manage this
+        'required'        => false, // Frontend will manage this based on country
         'priority'        => 85,
         'class'           => ['form-row-wide'],
         'placeholder'     => __('Search destination...', 'woonoow'),
@@ -122,6 +121,12 @@ add_filter('woocommerce_checkout_fields', function($fields) {
         ],
     ];
     
+    // Add to billing (used when "Ship to different address" is NOT checked)
+    $fields['billing']['billing_destination_id'] = $destination_field;
+    
+    // Add to shipping (used when "Ship to different address" IS checked)
+    $fields['shipping']['shipping_destination_id'] = $destination_field;
+    
     return $fields;
 }, 20); // Priority 20 to run after Rajaongkir's own filter
 

customer-spa/src/components/DynamicCheckoutField.tsx

@@ -162,7 +162,9 @@ export function DynamicCheckoutField({
                                 document.dispatchEvent(event);
                             }
                         }}
-                        placeholder={isSearching ? 'Searching...' : (field.placeholder || `Search ${field.label}...`)}
+                        onSearch={handleApiSearch}
+                        isSearching={isSearching}
+                        placeholder={field.placeholder || `Search ${field.label}...`}
                         emptyLabel={
                             isSearching
                                 ? 'Searching...'

customer-spa/src/components/ui/searchable-select.tsx

@@ -28,6 +28,9 @@ interface Props {
     emptyLabel?: string;
     className?: string;
     disabled?: boolean;
+    // For API-based search
+    onSearch?: (searchTerm: string) => void;
+    isSearching?: boolean;
 }
 
 export function SearchableSelect({
@@ -38,12 +41,26 @@ export function SearchableSelect({
     emptyLabel = "No results found.",
     className,
     disabled = false,
+    onSearch,
+    isSearching = false,
 }: Props) {
     const [open, setOpen] = React.useState(false);
+    const [searchValue, setSearchValue] = React.useState("");
     const selected = options.find((o) => o.value === value);
 
     React.useEffect(() => { if (disabled && open) setOpen(false); }, [disabled, open]);
 
+    // Handle search input changes
+    const handleSearchChange = (value: string) => {
+        setSearchValue(value);
+        if (onSearch) {
+            onSearch(value);
+        }
+    };
+
+    // Determine if we should use local filtering or API-based search
+    const shouldFilter = !onSearch;
+
     return (
         <Popover open={disabled ? false : open} onOpenChange={(o) => !disabled && setOpen(o)}>
             <PopoverTrigger asChild>
@@ -69,10 +86,16 @@ export function SearchableSelect({
                 align="start"
                 sideOffset={4}
             >
-                <Command shouldFilter>
-                    <CommandInput placeholder="Search..." />
+                <Command shouldFilter={shouldFilter}>
+                    <CommandInput
+                        placeholder="Search..."
+                        value={searchValue}
+                        onValueChange={handleSearchChange}
+                    />
                     <CommandList>
-                        <CommandEmpty>{emptyLabel}</CommandEmpty>
+                        <CommandEmpty>
+                            {isSearching ? "Searching..." : emptyLabel}
+                        </CommandEmpty>
                         {options.map((opt) => (
                             <CommandItem
                                 key={opt.value}
@@ -80,6 +103,7 @@ export function SearchableSelect({
                                 onSelect={() => {
                                     onChange?.(opt.value);
                                     setOpen(false);
+                                    setSearchValue("");
                                 }}
                             >
                                 <Check