DEV: Add `topic-list-header-sortable-column` transformer (#29917)

For conditionally disabling ability to sort on specific columns in the topic list.

app/assets/javascripts/discourse/app/components/topic-list/header.gjs

@@ -1,8 +1,15 @@
+import { hash } from "@ember/helper";
+import { applyValueTransformer } from "discourse/lib/transformer";
+
 const Header = <template>
   <tr>
     {{#each @columns as |entry|}}
       <entry.value.header
-        @sortable={{@sortable}}
+        @sortable={{applyValueTransformer
+          "topic-list-header-sortable-column"
+          @sortable
+          (hash category=@category name=@name)
+        }}
         @activeOrder={{@order}}
         @changeSort={{@changeSort}}
         @ascending={{@ascending}}

app/assets/javascripts/discourse/app/components/topic-list/list.gjs

@@ -121,10 +121,6 @@ export default class TopicList extends Component {
     return !this.bulkSelectEnabled && this.args.canBulkSelect;
   }
 
-  get sortable() {
-    return !!this.args.changeSort;
-  }
-
   get showTopicPostBadges() {
     return this.args.showTopicPostBadges ?? true;
   }
@@ -189,7 +185,7 @@ export default class TopicList extends Component {
           @order={{@order}}
           @changeSort={{@changeSort}}
           @ascending={{@ascending}}
-          @sortable={{this.sortable}}
+          @sortable={{@changeSort}}
           @listTitle={{or @listTitle "topic.title"}}
           @bulkSelectHelper={{@bulkSelectHelper}}
           @bulkSelectEnabled={{this.bulkSelectEnabled}}

app/assets/javascripts/discourse/app/lib/transformer/registry.js

@@ -16,4 +16,5 @@ export const VALUE_TRANSFORMERS = Object.freeze([
   "post-menu-buttons",
   "small-user-attrs",
   "topic-list-columns",
+  "topic-list-header-sortable-column",
 ]);