fix: use explicit Eastern timezone for day boundary instead of system TZ

getTodayLocal() relied on system clock hours, which broke in the web
container (TZ defaulting to UTC) — the day flipped at 11 PM EDT (3 AM
UTC) instead of 3 AM Eastern. Now uses Intl.DateTimeFormat with an
explicit America/New_York timezone. Also replaced all toISOString()
date formatting with local-component helpers to avoid UTC conversion.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

components/WeekNav.tsx

@@ -25,10 +25,17 @@ function formatLabel(dates: string[]): string {
   return `${startStr} – ${endStr}`;
 }
 
+function formatDateLocal(d: Date): string {
+  const y = d.getFullYear();
+  const m = String(d.getMonth() + 1).padStart(2, "0");
+  const day = String(d.getDate()).padStart(2, "0");
+  return `${y}-${m}-${day}`;
+}
+
 function shiftWeek(weekStart: string, delta: number): string {
   const d = new Date(weekStart + "T00:00:00");
   d.setDate(d.getDate() + delta * 7);
-  return d.toISOString().slice(0, 10);
+  return formatDateLocal(d);
 }
 
 export function WeekNav({ weekStart, weekDates, isCurrentWeek }: WeekNavProps) {