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/mix/tasks/generate_admin_icons.ex

@@ -43,7 +43,11 @@ defmodule Mix.Tasks.GenerateAdminIcons do
     content = """
     /* Generated by mix generate_admin_icons — do not edit by hand */
 
+    @layer admin {
+
     #{Enum.join(css_rules, "\n\n")}
+
+    } /* @layer admin */
     """
 
     File.mkdir_p!(Path.dirname(@output_path))