refactor: store selected week in a cookie, not the URL

The home page no longer reads ?week=YYYY-MM-DD from the URL. Selected week
lives in the tcWeek cookie, set via a server action that revalidates the
home page so the next render reflects it. The URL stays at "/" regardless
of which week the user is viewing.

WeekNav prev/next/today buttons (and the arrow-key bindings) call the
server action directly — no router.refresh dance, no client-side cookie
write. BackToCalendarLink drops its localStorage-based href reconstruction
and just links to "/" since the cookie already remembers the right week
across navigations.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

components/BackToCalendarLink.tsx

@@ -1,19 +1,12 @@
-"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}`);
-  }, []);
-
+  // The selected week is stored in a server-readable cookie (tcWeek), so the
+  // home page already renders the right week without us needing to pass it
+  // through the URL.
   return (
     <Link
-      href={href}
+      href="/"
       className="park-name-link"
       style={{
         display: "flex",