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>

app/page.tsx

@@ -1,5 +1,5 @@
 import { HomePageClient } from "@/components/HomePageClient";
-import { getTodayLocal } from "@/lib/env";
+import { getTodayLocal, formatDateLocal } from "@/lib/env";
 
 const BACKEND_URL = process.env.BACKEND_URL ?? "http://localhost:3001";
 
@@ -12,13 +12,13 @@ function getWeekStart(param: string | undefined): string {
     const d = new Date(param + "T00:00:00");
     if (!isNaN(d.getTime())) {
       d.setDate(d.getDate() - d.getDay());
-      return d.toISOString().slice(0, 10);
+      return formatDateLocal(d);
     }
   }
   const todayIso = getTodayLocal();
   const d = new Date(todayIso + "T00:00:00");
   d.setDate(d.getDate() - d.getDay());
-  return d.toISOString().slice(0, 10);
+  return formatDateLocal(d);
 }
 
 export default async function HomePage({ searchParams }: PageProps) {