topic_list: Set typeahead to only match when the prefix matches.

web/src/topic_list.ts

@@ -564,7 +564,7 @@ export function setup_topic_search_typeahead(): void {
             return typeahead_helper.render_topic_state(item);
         },
         matcher(item: string, query: string) {
-            return item.toLowerCase().includes(query.toLowerCase());
+            return item.toLowerCase().startsWith(query.toLowerCase());
         },
         sorter(items: string[]) {
             return items;