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/router.ex

@@ -138,9 +138,12 @@ defmodule BerrypodWeb.Router do
     get "/analytics/export", AnalyticsExportController, :export
     get "/newsletter/export", NewsletterExportController, :export
 
-    # No-JS fallbacks for email settings
+    # No-JS fallbacks for settings forms
     post "/settings/email", EmailSettingsController, :update
     post "/settings/email/test", EmailSettingsController, :test
+    post "/settings/from-address", SettingsController, :update_from_address
+    post "/settings/stripe/signing-secret", SettingsController, :update_signing_secret
+    post "/navigation", NavigationController, :save
 
     live_session :admin,
       layout: {BerrypodWeb.Layouts, :admin},