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/components/modal.js

@@ -591,6 +591,7 @@ function _validateField(input) {
   const hasValue = input.value.trim().length > 0;
   group?.classList.toggle('form-field--error', !hasValue);
   group?.classList.toggle('form-field--valid', hasValue);
+  input.setAttribute('aria-invalid', String(!hasValue));
   return hasValue;
 }