narrow: Add ApiNarrowIsUnread

Author: sirpengi

lib/api/model/narrow.dart

@@ -114,6 +114,14 @@ class ApiNarrowPmWith extends ApiNarrowDm {
   ApiNarrowPmWith._(super.operand, {super.negated});
 }
 
+// TODO: generalize into ApiNarrowIs
+class ApiNarrowIsUnread extends ApiNarrowElement {
+  @override String get operator => 'is';
+  @override String get operand => 'unread';
+
+  ApiNarrowIsUnread({super.negated});
+}
+
 class ApiNarrowMessageId extends ApiNarrowElement {
   @override String get operator => 'id';
 

lib/model/internal_link.dart

@@ -84,6 +84,8 @@ Uri narrowLink(PerAccountStore store, Narrow narrow, {int? nearMessageId}) {
         fragment.write('${element.operand.join(',')}-$suffix');
       case ApiNarrowDm():
         assert(false, 'ApiNarrowDm should have been resolved');
+      case ApiNarrowIsUnread():
+        fragment.write(element.operand.toString());
       case ApiNarrowMessageId():
         fragment.write(element.operand.toString());
     }