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/db.ts

@@ -1,6 +1,7 @@
 import Database from "better-sqlite3";
 import path from "path";
 import type { SiteContent, TeamMember, RichListItem, VictoryItem } from "@/types/content";
+import { MS_PER_DAY } from "@/lib/constants";
 
 const DB_PATH =
   process.env.DATABASE_PATH ||
@@ -728,7 +729,7 @@ export function setGroupBookingStatus(
   if (status === "confirmed" && confirmation) {
     // Auto-set reminder to 'coming' only if confirmed for today/tomorrow
     const today = new Date().toISOString().split("T")[0];
-    const tomorrow = new Date(Date.now() + 86400000).toISOString().split("T")[0];
+    const tomorrow = new Date(Date.now() + MS_PER_DAY).toISOString().split("T")[0];
     const reminderStatus = (confirmation.date === today || confirmation.date === tomorrow) ? "coming" : null;
     db.prepare(
       "UPDATE group_bookings SET status = ?, confirmed_date = ?, confirmed_group = ?, confirmed_comment = ?, notified_confirm = 1, reminder_status = ? WHERE id = ?"