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>

web/css/components/app-tooltips.css

@@ -99,4 +99,11 @@
   left: auto;
   transform: translate(calc(-1 * (var(--tooltip-offset) - 2px)), -50%) rotate(45deg);
 }
+
+@media (prefers-reduced-motion: reduce) {
+  [data-tooltip]::after,
+  [data-tooltip]::before {
+    transition: none;
+  }
+}
 }