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/components/core_components.ex

@@ -86,7 +86,8 @@ defmodule BerrypodWeb.CoreComponents do
     <span
       :if={@status != :idle}
       class={["admin-inline-feedback", "admin-inline-feedback-#{@status}"]}
-      role={@status == :error && "alert"}
+      role={if(@status == :error, do: "alert", else: "status")}
+      aria-live={if(@status == :error, do: "assertive", else: "polite")}
     >
       <.icon :if={@status == :saving} name="hero-arrow-path" class="size-4 motion-safe:animate-spin" />
       <.icon :if={@status == :saved} name="hero-check" class="size-4" />