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>

mix.exs

@@ -62,6 +62,7 @@ defmodule Berrypod.MixProject do
        compile: false,
        depth: 1},
       {:swoosh, "~> 1.16"},
+      {:gen_smtp, "~> 1.0"},
       {:req, "~> 0.5"},
       {:telemetry_metrics, "~> 1.0"},
       {:telemetry_poller, "~> 1.0"},