feat: amber indicator during post-close wind-down buffer

Parks in the 1-hour buffer after scheduled close now show amber instead
of green: the dot on the desktop calendar turns yellow, and the mobile
card badge changes from "Open today" (green) to "Closing" (amber).

- getOperatingStatus() replaces isWithinOperatingWindow's inline logic,
  returning "open" | "closing" | "closed"; isWithinOperatingWindow now
  delegates to it so all callers are unchanged
- closingParkIds[] is computed server-side and threaded through
  HomePageClient → WeekCalendar/MobileCardList → ParkRow/ParkCard
- New --color-closing-* CSS variables mirror the green palette in amber

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

app/globals.css

@@ -27,6 +27,12 @@
   --color-open-text: #4ade80;
   --color-open-hours: #bbf7d0;
 
+  /* ── Closing — amber (post-close buffer, rides still winding down) ───────── */
+  --color-closing-bg: #1a1100;
+  --color-closing-border: #d97706;
+  --color-closing-text: #fbbf24;
+  --color-closing-hours: #fde68a;
+
   /* ── Passholder preview — vivid cyan ─────────────────────────────────────── */
   --color-ph-bg: #051518;
   --color-ph-border: #22d3ee;