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.

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