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/js/components/app-toggle-sidebar.js

@@ -2,6 +2,7 @@
  * Sidebar collapse/show toggle with localStorage persistence and Ctrl+B shortcut.
  */
 import { requireEl, warnOnce } from '../core/app-dom.js';
+import { isEditableTarget } from '../core/app-form-utils.js';
 
 export function initSidebarToggle(options = {}) {
   const {
@@ -150,13 +151,6 @@ export function initSidebarToggle(options = {}) {
   return controller;
 }
 
-const isEditableTarget = (target) => {
-  if (!target) {return false;}
-  if (target.isContentEditable) {return true;}
-  const tag = target.tagName;
-  return tag === 'INPUT' || tag === 'TEXTAREA' || tag === 'SELECT';
-};
-
 document.addEventListener('keydown', (event) => {
   if (!(event.metaKey || event.ctrlKey)) {return;}
   if (event.altKey) {return;}