compose-closed: Fix updating buttons when not in DM narrow.

When processing a server event or an empty message fetch,
we were incorrectly updating the closed new conversation
button attribute, "data-conversation-type" to be "direct"
without checking to see if the current narrow filter was
actually a direct message view.

Adds a helper function that checks the current filter for
contains_only_private_messages and then calls
compose_closed_ui.update_buttons_for_private when true.

That's the same check that's used in
message_view.handle_post_view_change when
compose_closed_ui.update_buttons_for_private is called,
so we're consistently using the same check when updating
the closed compose box buttons for this state.

web/src/compose_closed_ui.ts

@@ -208,6 +208,13 @@ export function update_buttons_for_non_specific_views(): void {
     set_standard_text_for_reply_button();
 }
 
+export function maybe_update_buttons_for_dm_recipient(): void {
+    const filter = narrow_state.filter();
+    if (filter?.contains_only_private_messages()) {
+        update_buttons_for_private();
+    }
+}
+
 function set_reply_button_label(label: string): void {
     $("#left_bar_compose_reply_button_big").text(label);
 }

web/src/message_fetch.ts

@@ -128,7 +128,7 @@ export function fetch_more_if_required_for_current_msg_list(
         // no messages to display in this narrow.
         narrow_banner.show_empty_narrow_message(message_lists.current.data.filter);
         message_lists.current.update_trailing_bookend();
-        compose_closed_ui.update_buttons_for_private();
+        compose_closed_ui.maybe_update_buttons_for_dm_recipient();
         compose_recipient.check_posting_policy_for_compose_box();
     }
 

web/src/server_events_dispatch.js

@@ -408,7 +408,7 @@ export function dispatch_normal_event(event) {
                                     key === "direct_message_permission_group"
                                 ) {
                                     settings_org.check_disable_direct_message_initiator_group_widget();
-                                    compose_closed_ui.update_buttons_for_private();
+                                    compose_closed_ui.maybe_update_buttons_for_dm_recipient();
                                     compose_recipient.check_posting_policy_for_compose_box();
                                 }