Toast alert a11y v19

[Wagner3UB]

Accessibility improvements for Toast notifications

This PR improves the accessibility of the Toast component for screen readers and general UX.

Changes

Toast component (Toast.jsx)

• Icon is now hidden from screen readers (ariaHidden) since it is purely decorative
• Added a visually hidden label announcing the notification type (Error, Warning, Success, Information) so screen readers can identify the context before reading the content
• Labels are fully internationalized via react-intl (defineMessages + useIntl)

Styles (main.less)

• Added .toast-visually-hidden scoped inside .toast-inner-content to hide the type label visually while keeping it accessible to screen readers

Date formatting in event validation (validators.ts)

• Date values in event range error messages were being displayed as raw ISO strings (e.g. 2026-04-08T15:00:00+00:00)
• Now formatted using Intl.DateTimeFormat respecting the user's browser locale (e.g. Apr 8, 2026, 3:00 PM)

[Wagner3UB]

@sneridagh Hey Victor, could you take a look at the 'validations.js' file? I changed how the dates are displayed, but I'm not sure if it might cause issues since this file is used so frequently.

[stevepiercy]

LGTM, except for the local settings file.