fix: date validation, pre-fill on re-edit, MS_PER_DAY constant

- Extract MS_PER_DAY constant to lib/constants.ts
- ConfirmModal date: max=1 year, rejects past dates and malformed years
- ConfirmModal pre-fills existing date + group when re-editing (✎)
- Confirmed date display handles malformed dates gracefully
- Red border + error for invalid dates, submit disabled

src/lib/constants.ts

@@ -1,5 +1,7 @@
 import type { NavLink } from "@/types";
 
+export const MS_PER_DAY = 24 * 60 * 60 * 1000;
+
 export const BRAND = {
   name: "BLACK HEART DANCE HOUSE",
   shortName: "Blackheart",