DXtrade: date transactions by the firm's day, not the CT trade date

The firm rolls its day at 22:00 UTC; our trade date at 5 PM CT. In summer
they coincide, in winter the firm's rolls an hour earlier. New entries are
already kept out of that hour, but a take-profit or stop that fills in it
was booked to the day before. firmDayOf labels the firm's day the way
currentTradeDateCT labels ours, and the sync's transfer window now starts
at the firm day behind today's trade date.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/dxtrade-helpers.ts

@@ -91,6 +91,15 @@ export function lastUtcReset(now: Date, hourUtc = 22): Date {
     return reset;
 }
 
+/** The trading day the firm books a moment to: the day that began at the
+ *  reset before it, labelled by the calendar date that follows — the label
+ *  currentTradeDateCT gives the same day. In summer the two roll together
+ *  (22:00 UTC = 5 PM CDT); in winter the firm's rolls an hour earlier, and a
+ *  close in that hour belongs to the new day, as the firm counts it. */
+export function firmDayOf(at: Date, hourUtc = 22): string {
+    return new Date(at.getTime() + (24 - hourUtc) * 3_600_000).toISOString().slice(0, 10);
+}
+
 /** When the current Central trade date began (5 PM CT) — the window today's
  *  realized P&L covers, matching currentTradeDateCT. */
 export function ctTradeDayStart(now: Date): Date {