add admin page editor with block reordering and management

Stage 6 of the page builder: admin UI at /admin/pages for managing
page layouts. Page list shows all 14 pages grouped by category.
Editor supports reorder (up/down), add, remove, duplicate, save,
and reset to defaults. DirtyGuard JS hook warns on unsaved changes.
ARIA live regions announce block operations for screen readers.

Also: regenerate admin icons (81 rules via mix task with @layer
wrapping), add gen_smtp dep for SMTP email adapter, add :key to
page renderer block loop for correct LiveView diffing.

1309 tests, 0 failures.

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

lib/berrypod_web/components/layouts/admin.html.heex

@@ -94,6 +94,14 @@
               <.icon name="hero-link" class="size-5" /> Providers
             </.link>
           </li>
+          <li>
+            <.link
+              navigate={~p"/admin/pages"}
+              class={admin_nav_active?(@current_path, "/admin/pages")}
+            >
+              <.icon name="hero-document" class="size-5" /> Pages
+            </.link>
+          </li>
           <li>
             <.link
               href={~p"/admin/theme"}