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>

assets/js/app.js

@@ -634,10 +634,27 @@ const ChartTooltip = {
   }
 }
 
+// Warns before navigating away from pages with unsaved changes
+const DirtyGuard = {
+  mounted() {
+    this._beforeUnload = (e) => {
+      if (this.el.dataset.dirty === "true") {
+        e.preventDefault()
+        e.returnValue = ""
+      }
+    }
+    window.addEventListener("beforeunload", this._beforeUnload)
+  },
+
+  destroyed() {
+    window.removeEventListener("beforeunload", this._beforeUnload)
+  }
+}
+
 const csrfToken = document.querySelector("meta[name='csrf-token']").getAttribute("content")
 const liveSocket = new LiveSocket("/live", Socket, {
   params: {_csrf_token: csrfToken, screen_width: window.innerWidth},
-  hooks: {...colocatedHooks, ColorSync, Lightbox, CartPersist, CartDrawer, ProductImageScroll, SearchModal, CollectionFilters, CardRadioScroll, AnalyticsInit, AnalyticsExport, ChartTooltip},
+  hooks: {...colocatedHooks, ColorSync, Lightbox, CartPersist, CartDrawer, ProductImageScroll, SearchModal, CollectionFilters, CardRadioScroll, AnalyticsInit, AnalyticsExport, ChartTooltip, DirtyGuard},
 })
 
 // Show progress bar on live navigation and form submits