add admin CRUD for custom CMS pages

New settings form for creating and editing custom page metadata
(title, slug, meta description, published, nav settings). Pages
index shows custom pages section with draft badges and delete.
Editor shows settings button for custom pages, hides reset to
defaults. 20 new tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/berrypod_web/router.ex

@@ -155,6 +155,8 @@ defmodule BerrypodWeb.Router do
       live "/settings", Admin.Settings, :index
       live "/settings/email", Admin.EmailSettings, :index
       live "/pages", Admin.Pages.Index, :index
+      live "/pages/new", Admin.Pages.CustomForm, :new
+      live "/pages/:slug/settings", Admin.Pages.CustomForm, :edit
       live "/pages/:slug", Admin.Pages.Editor, :edit
       live "/media", Admin.Media, :index
       live "/redirects", Admin.Redirects, :index