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/pages/tasks.js

@@ -1092,6 +1092,13 @@ function updateOverdueBadge() {
   }).length;
 
   document.querySelectorAll('[data-route="/tasks"] .nav-badge').forEach((el) => el.remove());
+  document.querySelectorAll('[data-route="/tasks"]').forEach((navItem) => {
+    const baseLabel = t('tasks.title');
+    navItem.setAttribute('aria-label', overdue > 0
+      ? t('tasks.navLabelOverdue', { count: overdue })
+      : baseLabel
+    );
+  });
   if (overdue > 0) {
     document.querySelectorAll('[data-route="/tasks"]').forEach((navItem) => {
       let anchor = navItem.querySelector('.nav-item__icon-wrap');
@@ -1108,6 +1115,7 @@ function updateOverdueBadge() {
       }
       const badge = document.createElement('span');
       badge.className = 'nav-badge';
+      badge.setAttribute('aria-hidden', 'true');
       badge.textContent = String(overdue);
       anchor.appendChild(badge);
     });