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/page_renderer.ex

@@ -35,7 +35,7 @@ defmodule BerrypodWeb.PageRenderer do
       error_page={@page.slug == "error"}
     >
       <main id="main-content" class={page_main_class(@page.slug)}>
-        <div :for={block <- @page.blocks} data-block-type={block["type"]}>
+        <div :for={block <- @page.blocks} :key={block["id"]} data-block-type={block["type"]}>
           {render_block(Map.merge(@block_assigns, %{block: block, page_slug: @page.slug}))}
         </div>
       </main>