combined-feed-ui: Add placeholder text to empty combined feed.

Also, added live update behaviour to the combined feed narrow.

Fixes #31602.

web/src/narrow_banner.ts

@@ -128,10 +128,28 @@ export function pick_empty_narrow_banner(): NarrowBannerData {
     const default_banner_for_multiple_filters = $t({defaultMessage: "No search results."});
 
     const current_filter = narrow_state.filter();
-
-    if (current_filter === undefined || current_filter.is_in_home()) {
+    if (current_filter === undefined) {
+        // We're in either the inbox or recent conversations view.
         return default_banner;
     }
+    if (current_filter.is_in_home()) {
+        // We're in the combined feed view.
+        return {
+            title: $t({defaultMessage: "There are no messages in your combined feed."}),
+            html: page_params.is_spectator
+                ? ""
+                : $t_html(
+                      {
+                          defaultMessage:
+                              "Would you like to <z-link>view messages in all public channels</z-link>?",
+                      },
+                      {
+                          "z-link": (content_html) =>
+                              `<a href="#narrow/channels/public">${content_html.join("")}</a>`,
+                      },
+                  ),
+        };
+    }
 
     const first_term = current_filter.terms()[0]!;
     const current_terms_types = current_filter.sorted_term_types();