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/WeekNav.tsx

@@ -1,7 +1,7 @@
 "use client";
 
 import { useEffect } from "react";
-import { useRouter } from "next/navigation";
+import { setWeek, clearWeek } from "@/app/actions/week";
 
 interface WeekNavProps {
   weekStart: string;   // YYYY-MM-DD (Sunday)
@@ -39,9 +39,11 @@ function shiftWeek(weekStart: string, delta: number): string {
 }
 
 export function WeekNav({ weekStart, weekDates, isCurrentWeek }: WeekNavProps) {
-  const router = useRouter();
   const nav = (delta: number) => {
-    router.push(`/?week=${shiftWeek(weekStart, delta)}`);
+    void setWeek(shiftWeek(weekStart, delta));
+  };
+  const jumpToToday = () => {
+    void clearWeek();
   };
 
   useEffect(() => {
@@ -68,7 +70,7 @@ export function WeekNav({ weekStart, weekDates, isCurrentWeek }: WeekNavProps) {
 
       {!isCurrentWeek && (
         <button
-          onClick={() => router.push("/")}
+          onClick={jumpToToday}
           aria-label="Jump to current week"
           style={todayBtnStyle}
           onMouseOver={(e) => Object.assign((e.target as HTMLElement).style, todayBtnHover)}