feat(user-settings): mark beacon monitors as not yet working

The monitor client and ingestion pipeline aren't shipped, so warn
users that registering a monitor here doesn't do anything yet.

lib/microwaveprop_web/controllers/user_settings_html/edit.html.heex

@@ -58,6 +58,11 @@
       </:subtitle>
     </.header>
 
+    <div class="alert alert-warning text-sm">
+      <.icon name="hero-exclamation-triangle" class="size-4" />
+      <span>Not working yet — monitor client + ingestion pipeline still in development.</span>
+    </div>
+
     <.form
       :let={f}
       for={@beacon_monitor_changeset}