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/ARCHITECTURE.md

@@ -420,12 +420,18 @@ park/[id]/page.tsx (Server)
 2. **Opening-time refresh**: For each park open today, calculates milliseconds until its opening time using `msUntilLocalTime()` (timezone-aware). Schedules `router.refresh()` at opening and again 30 seconds later (to pick up ride counts after Queue-Times starts reporting).
 3. **Keyboard navigation**: Left/right arrow keys navigate between weeks (via `WeekNav` component).
 
+### Cookies
+
+| Name | Purpose |
+|------|---------|
+| `tcWeek` | Selected week start date (`YYYY-MM-DD`). Set by `WeekNav` and read server-side by `app/page.tsx`, so the home page renders the right week without polluting the URL. |
+
 ### localStorage
 
 | Key | Purpose |
 |-----|---------|
-| `lastWeek` | Remembers the last viewed week start date, used by `BackToCalendarLink` to return to the correct week |
 | `coasterMode` | Persists the "Coasters only" toggle state across sessions |
+| `fastLaneMode` | Persists the Fast Lane wait toggle on the park page |
 
 ### Responsive Design