fix: "← Calendar" returns to the previously viewed week

HomePageClient writes the current weekStart to localStorage("lastWeek")
whenever it changes. BackToCalendarLink (new client component) reads
that value on mount and builds the href — falling back to "/" if nothing
is stored (e.g. direct link to a park page).

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

app/park/[id]/page.tsx

@@ -1,4 +1,5 @@
 import Link from "next/link";
+import { BackToCalendarLink } from "@/components/BackToCalendarLink";
 import { notFound } from "next/navigation";
 import { PARK_MAP } from "@/lib/parks";
 import { openDb, getParkMonthData, getApiId } from "@/lib/db";
@@ -93,19 +94,7 @@ export default async function ParkPage({ params, searchParams }: PageProps) {
         alignItems: "center",
         gap: 16,
       }}>
-        <Link href="/" style={{
-          display: "flex",
-          alignItems: "center",
-          gap: 6,
-          fontSize: "0.8rem",
-          color: "var(--color-text-muted)",
-          textDecoration: "none",
-          transition: "color 120ms ease",
-        }}
-          className="park-name-link"
-        >
-          ← Calendar
-        </Link>
+        <BackToCalendarLink />
         <div style={{ width: 1, height: 16, background: "var(--color-border)" }} />
         <span style={{ fontSize: "0.9rem", fontWeight: 600, color: "var(--color-text)", letterSpacing: "-0.01em" }}>
           {park.name}

components/BackToCalendarLink.tsx

@@ -0,0 +1,31 @@
+"use client";
+
+import { useState, useEffect } from "react";
+import Link from "next/link";
+
+export function BackToCalendarLink() {
+  const [href, setHref] = useState("/");
+
+  useEffect(() => {
+    const saved = localStorage.getItem("lastWeek");
+    if (saved) setHref(`/?week=${saved}`);
+  }, []);
+
+  return (
+    <Link
+      href={href}
+      className="park-name-link"
+      style={{
+        display: "flex",
+        alignItems: "center",
+        gap: 6,
+        fontSize: "0.8rem",
+        color: "var(--color-text-muted)",
+        textDecoration: "none",
+        transition: "color 120ms ease",
+      }}
+    >
+      ← Calendar
+    </Link>
+  );
+}

components/HomePageClient.tsx

@@ -41,6 +41,11 @@ export function HomePageClient({
     setCoastersOnly(localStorage.getItem(COASTER_MODE_KEY) === "true");
   }, []);
 
+  // Remember the current week so the park page back button returns here.
+  useEffect(() => {
+    localStorage.setItem("lastWeek", weekStart);
+  }, [weekStart]);
+
   const toggle = () => {
     const next = !coastersOnly;
     setCoastersOnly(next);