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>

docs/DEVELOPMENT.md

@@ -55,7 +55,7 @@ npm run dev
 This starts the Next.js dev server on port 3000 with hot reload. Open [http://localhost:3000](http://localhost:3000).
 
 **Navigation:**
-- Use the `←` / `→` buttons to navigate weeks, or pass `?week=YYYY-MM-DD` in the URL
+- Use the `←` / `→` buttons (or arrow keys) to navigate weeks; the selected week persists across visits via the `tcWeek` cookie
 - Click any park name to open its detail page with month calendar and ride status
 
 ---