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/controllers/settings_controller.ex

@@ -0,0 +1,42 @@
+defmodule BerrypodWeb.SettingsController do
+  @moduledoc """
+  No-JS fallback for settings form submissions.
+
+  With JS enabled, the LiveView handles everything. Without JS,
+  the forms POST here and we redirect back to the LiveView page.
+  """
+  use BerrypodWeb, :controller
+
+  alias Berrypod.Settings
+  alias Berrypod.Stripe.Setup, as: StripeSetup
+
+  def update_from_address(conn, %{"from_address" => address}) do
+    address = String.trim(address)
+
+    if address != "" do
+      Settings.put_setting("email_from_address", address)
+
+      conn
+      |> put_flash(:info, "From address saved")
+      |> redirect(to: ~p"/admin/settings")
+    else
+      conn
+      |> put_flash(:error, "From address can't be blank")
+      |> redirect(to: ~p"/admin/settings")
+    end
+  end
+
+  def update_signing_secret(conn, %{"webhook" => %{"signing_secret" => secret}}) do
+    if secret == "" do
+      conn
+      |> put_flash(:error, "Please enter a signing secret")
+      |> redirect(to: ~p"/admin/settings")
+    else
+      StripeSetup.save_signing_secret(secret)
+
+      conn
+      |> put_flash(:info, "Signing secret saved")
+      |> redirect(to: ~p"/admin/settings")
+    end
+  end
+end