feat(a11y): WCAG 2.2 accessibility fixes across four areas

- modal/_validateField: set aria-invalid on invalid inputs so screen readers
  announce field errors; login.js mirrors this for username/password fields
- color pickers (notes, calendar): wrap swatches in role="radiogroup" with
  aria-labelledby, add aria-checked per swatch, localized aria-labels instead
  of hex values, roving tabindex with Arrow/Enter/Space keyboard navigation
- nav badges: badge spans get aria-hidden="true"; nav link aria-label updated
  to include overdue count (tasks) or pending reminder count (reminders)
- router: remove aria-live from <main> (caused full page re-reads on nav);
  add dedicated #route-announcer sr-only region with aria-live=polite +
  aria-atomic, announces page label 50ms after render completes

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

public/reminders.js

@@ -69,8 +69,15 @@ function showBrowserNotification(title, body) {
  * @param {number} count
  */
 function updateBellBadge(count) {
+  const navLabel = count > 0
+    ? t(count === 1 ? 'reminders.pendingBadgeTitle' : 'reminders.pendingBadgeTitlePlural', { count })
+    : t('nav.reminders');
+  document.querySelectorAll('[data-route="/reminders"]').forEach((navItem) => {
+    navItem.setAttribute('aria-label', navLabel);
+  });
   document.querySelectorAll('.reminder-bell-badge').forEach((badge) => {
     if (count > 0) {
+      badge.setAttribute('aria-hidden', 'true');
       badge.textContent = count > 9 ? '9+' : String(count);
       badge.hidden = false;
     } else {