refactor(ui): move toast stack chrome to a server-rendered partial

Mirrors the detail-drawer move: the .app-toast-stack container is no
longer lazy-created in JS on first toast, instead it lives in
templates/partials/app-toast-stack.phtml and is mounted once in
default.phtml alongside the confirm dialog. The aria-live attribute now
sits in the SSR markup so screen readers register the live region from
page load, not from the first toast.

getToastStack() drops to a plain querySelector and warns via warnOnce
when the partial is missing instead of constructing a fallback container.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

templates/default.phtml

@@ -337,6 +337,7 @@ $componentPageConfig['moduleRuntimeComponents'] = $moduleRuntimeComponents;
 
   <script src="<?php e(assetVersion('vendor/fslightbox/fslightbox.js')); ?>"></script>
   <?php require __DIR__ . '/partials/app-confirm-dialog.phtml'; ?>
+  <?php require __DIR__ . '/partials/app-toast-stack.phtml'; ?>
   <?php if ($isLoggedIn): ?>
   <?php require __DIR__ . '/partials/app-search-dialog.phtml'; ?>
   <?php require __DIR__ . '/partials/app-session-warning-dialog.phtml'; ?>

templates/partials/app-toast-stack.phtml

@@ -0,0 +1,12 @@
+<?php
+
+/**
+ * Async toast stack — global container for JS-triggered notifications.
+ *
+ * Mounted once at the bottom of the shell so any page can fire toasts via
+ * showAsyncFlash() without the JS having to lazy-create the container.
+ * Empty by default; toasts are appended/removed by web/js/components/app-async-flash.js.
+ */
+
+?>
+<div class="app-toast-stack" aria-live="polite"></div>