refactor: extract shared form utils, add aria-live regions and reduced-motion support

DRY duplicated helpers (escapeAttr, belongsToForm, isSubmitControl,
isEditableTarget) into web/js/core/app-form-utils.js and update 7
consumers. Add aria-live="polite" to flash messages, async messages,
and search results for screen reader announcements. Add
prefers-reduced-motion support to CSS tooltips.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

templates/partials/app-flash.phtml

@@ -18,7 +18,7 @@ $timeouts = [
 ];
 
 ?>
-<div class="flash-stack">
+<div class="flash-stack" role="status" aria-live="polite">
   <?php foreach ($messages as $message) : ?>
     <?php $type = $message['type'] ?? 'info'; ?>
     <?php $timeout = $timeouts[$type] ?? 5000; ?>

templates/partials/app-main-aside.phtml

@@ -480,7 +480,7 @@ $csrfToken = (string) ($layoutNav['csrfToken'] ?? '');
                 <small class="muted" data-search-empty-hint-template="<?php e(t('Tip: Use {shortcut} to focus search quickly.')); ?>"></small>
             </div>
             <nav id="global-search">
-                <ul class="app-search-results" data-global-search-results>
+                <ul class="app-search-results" data-global-search-results aria-live="polite">
                     <?php if ($canViewUsers): ?>
                         <?php $usersSearch = navActive('admin/users', true); ?>
                         <li data-search-key="users" data-search-base="<?php e(lurl('admin/users')); ?>">

templates/partials/app-topbar.phtml

@@ -32,7 +32,7 @@ $allowUserTheme = allowUserTheme();
       <li>
         <a id="global-forward" data-tooltip-pos="bottom" data-tooltip="<?php e(t('Forward')); ?> (Alt+→)" title="<?php e(t('Forward')); ?> (Alt+→)" aria-label="<?php e(t('Forward')); ?> (Alt+→)" href="admin"><i class="bi bi-arrow-right"></i></a>
       </li>
-      <li id="async-messages"></li>
+      <li id="async-messages" role="status" aria-live="polite"></li>
     </ul>
     <div class="app-topbar-center" aria-hidden="true"></div>
     <ul class="app-topbar-right">