add link picker and validation to navigation editor

- replace freeform inputs with grouped dropdown (pages, custom pages,
  collections, external URL)
- add inline URL validation for external links
- add inline feedback component instead of flash messages
- add dismiss-on-interaction pattern (feedback clears on changes)
- add no-JS fallback via NavigationController
- add DirtyGuard hook to warn before navigating away with unsaved changes
- add no-JS fallbacks for settings forms (from address, signing secret)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/berrypod_web/live/admin/email_settings.ex

@@ -68,7 +68,7 @@ defmodule BerrypodWeb.Admin.EmailSettings do
   @impl true
   def handle_event("form_change", %{"email" => %{"adapter" => key}}, socket) do
     if key == socket.assigns.adapter_key do
-      {:noreply, socket}
+      {:noreply, assign(socket, :save_status, :idle)}
     else
       {:noreply,
        socket
@@ -76,7 +76,8 @@ defmodule BerrypodWeb.Admin.EmailSettings do
        |> assign(:selected_adapter, Adapters.get(key))
        |> assign(:field_errors, %{})
        |> assign(:test_result, nil)
-       |> assign(:test_error, nil)}
+       |> assign(:test_error, nil)
+       |> assign(:save_status, :idle)}
     end
   end
 
@@ -94,8 +95,6 @@ defmodule BerrypodWeb.Admin.EmailSettings do
              socket.assigns.current_scope.user.email
            ) do
         {:ok, _adapter_info} ->
-          Process.send_after(self(), :clear_save_status, 3000)
-
           {:noreply,
            socket
            |> assign(:adapter_key, adapter_key)
@@ -145,10 +144,6 @@ defmodule BerrypodWeb.Admin.EmailSettings do
   # Swoosh test adapter sends {:email, ...} messages — ignore them
   def handle_info({:email, _}, socket), do: {:noreply, socket}
 
-  def handle_info(:clear_save_status, socket) do
-    {:noreply, assign(socket, :save_status, :idle)}
-  end
-
   @impl true
   def render(assigns) do
     ~H"""