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/pages/editor.ex

@@ -384,8 +384,6 @@ defmodule BerrypodWeb.Admin.Pages.Editor do
 
     case Pages.save_page(slug, %{title: page_data.title, blocks: blocks}) do
       {:ok, _page} ->
-        Process.send_after(self(), :clear_save_status, 3000)
-
         {:noreply,
          socket
          |> assign(:dirty, false)
@@ -523,11 +521,6 @@ defmodule BerrypodWeb.Admin.Pages.Editor do
 
   # ── Handle info ──────────────────────────────────────────────────
 
-  @impl true
-  def handle_info(:clear_save_status, socket) do
-    {:noreply, assign(socket, :save_status, :idle)}
-  end
-
   # ── Render ───────────────────────────────────────────────────────
 
   @impl true
@@ -857,6 +850,7 @@ defmodule BerrypodWeb.Admin.Pages.Editor do
     |> assign(:history, history)
     |> assign(:future, [])
     |> assign(:dirty, true)
+    |> assign(:save_status, :idle)
     |> assign(:live_region_message, message)
   end
 end