Improve API listing ID handling and add buyer note feature

lib/api/generated/api/catalog_api.dart

@@ -420,7 +420,7 @@ class CatalogApi {
   /// Carries the variants with their prices and stock and the seller summary.  A listing that was never public — `draft` or `pending` — is readable only by its owner and by staff. One that is `hidden` or soft-deleted is readable by anyone, because a cart or an order that references it still has to render; `status` and `deleted_at` say that it cannot be bought.
   ///
   /// Parameters:
-  /// * [id] - The listing's opaque id or its slug. A slug is lowercase, hyphen-separated and never contains an underscore, so it cannot be mistaken for an id — but only the id is accepted today; resolving a slug needs a lookup this route does not have yet.
+  /// * [id] - The listing's opaque id or the slug a link carries. A slug is lowercase, hyphen-separated and never contains an underscore, so it cannot be mistaken for an id; it resolves without a lookup, because it carries the id on the end.
   /// * [cancelToken] - A [CancelToken] that can be used to cancel the operation
   /// * [headers] - Can be used to add additional headers to the request
   /// * [extras] - Can be used to add flags to the request
@@ -502,7 +502,7 @@ class CatalogApi {
   }
 
   /// Edit a listing
-  /// An edit to a draft applies immediately. An edit to a live listing that trips the prohibited-goods filter is held as a pending edit and applied only when a moderator approves it, so the version buyers see never changes into unreviewed content.  The slug is fixed at creation. Renaming a listing leaves it alone: there is no redirect table, so a new slug would only turn every link already shared into a 404, and `GET /listings/{id}` takes the id as well.
+  /// An edit to a draft applies immediately. An edit to a live listing that trips the prohibited-goods filter is held as a pending edit and applied only when a moderator approves it, so the version buyers see never changes into unreviewed content.  The slug's readable half is fixed at creation. Renaming a listing leaves it alone, so a link shared before the rename keeps saying what it said then — and it would resolve either way, since the id it carries is what addresses the listing.
   ///
   /// Parameters:
   /// * [id] - The opaque id. A write addresses the listing, never its public slug.

lib/api/generated/deserialize.dart

@@ -231,8 +231,6 @@ ReturnType deserialize<ReturnType, BaseType>(
   bool growable = true,
 }) {
   switch (targetType) {
-    case 'String':
-      return '$value' as ReturnType;
     case 'int':
       return (value is int ? value : int.parse('$value')) as ReturnType;
     case 'bool':

lib/api/generated/model/listing.dart

@@ -129,7 +129,7 @@ class Listing {
   @JsonKey(name: r'seller', required: true, includeIfNull: false)
   final AccountSummary seller;
 
-  /// URL-friendly, globally unique and fixed at creation.
+  /// The slug a link carries: the listing's name, slugified and frozen at creation, with the listing's id on the end. The text in front is for the person reading the link and is not matched — renaming a listing does not rewrite links already shared, and a stale one still resolves because the id behind the last hyphen is what addresses the listing.  It never contains an underscore, which is how `GET /listings/{id}` tells it from a `ListingID` and accepts either. A name that slugifies to nothing leaves the id alone.
   @JsonKey(name: r'slug', required: true, includeIfNull: false)
   final String slug;
 

lib/api/generated/model/listing_detail.dart

@@ -146,6 +146,7 @@ class ListingDetail {
   @JsonKey(name: r'seller', required: true, includeIfNull: false)
   final AccountSummary seller;
 
+  /// The slug a link carries: the listing's name, slugified and frozen at creation, with the listing's id on the end. The text in front is for the person reading the link and is not matched — renaming a listing does not rewrite links already shared, and a stale one still resolves because the id behind the last hyphen is what addresses the listing.  It never contains an underscore, which is how `GET /listings/{id}` tells it from a `ListingID` and accepts either. A name that slugifies to nothing leaves the id alone.
   @JsonKey(name: r'slug', required: true, includeIfNull: false)
   final String slug;
 

lib/features/checkout/presentation/providers/checkout_provider.dart

@@ -65,6 +65,7 @@ abstract class CheckoutState with _$CheckoutState {
     /// checkout that breaks the day an operator retires that rail.
     @Default([]) List<Option> paymentOptions,
     String? paymentOption,
+    String? note,
     CheckoutResult? checkoutResult,
     Transaction? paymentTransaction,
     PaymentSession? paymentSession,
@@ -385,6 +386,11 @@ class CheckoutNotifier extends _$CheckoutNotifier {
     }
   }
 
+  /// Cập nhật lời nhắn cho người bán
+  void setNote(String note) {
+    state = state.copyWith(note: note);
+  }
+
   /// Thay đổi trạng thái đồng ý điều khoản dịch vụ
   void toggleAgreeToTerms({bool? value}) {
     state = state.copyWith(
@@ -424,6 +430,9 @@ class CheckoutNotifier extends _$CheckoutNotifier {
             for (final line in state.lines)
               CheckoutLine(variantId: line.variantId, quantity: line.quantity),
           ],
+          note: (state.note?.trim().isNotEmpty == true)
+              ? state.note!.trim()
+              : null,
           transportOption: state.transportOption!,
         ),
       );