Default event form dates to today with end matching start

[dgershman]

Closes #301

What & why

The public event submission form ([mayo_event_form]) started with blank Start Date and End Date fields. Issue #301 asked to default the End Date to the Start Date and to default the start year to the current year.

Changes (assets/js/src/components/public/EventForm.js)

• Start Date pre-fills to today's date (delivers the current year; a native <input type="date"> can't default only the year). Uses a local-time getTodayLocalDate() helper — not toISOString() — to avoid a UTC off-by-one.
• End Date matches Start Date out of the box, and follows it whenever the Start Date changes — but only while the two are still equal, so a deliberately different multi-day end date is never clobbered.
• The post-submit form reset restores the same today-based defaults.

No PHP changed. readme.txt changelog updated under the unreleased = 1.9.2 = section (no version bump). The package-lock.json diff is just a stale 1.9.1 → 1.9.2 version sync with package.json.

Testing

• npm run build compiles cleanly.
• Manual: on load both dates show today; changing Start Date updates End Date to match; after setting a later End Date, changing Start Date leaves it intact; submitting resets both to today.