feat: coaster filter toggle on homepage

- 🎢 Coasters button in nav bar (URL-driven: ?coasters=1)
- When active, swaps ride counts for coaster counts per park
- Label switches between "X rides operating" / "X coasters operating"
- Arrow key navigation preserves coaster filter state
- Only shown when coaster data exists in park-meta

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

components/WeekNav.tsx

@@ -7,6 +7,8 @@ interface WeekNavProps {
   weekStart: string;   // YYYY-MM-DD (Sunday)
   weekDates: string[]; // 7 dates YYYY-MM-DD
   isCurrentWeek: boolean;
+  coastersOnly: boolean;
+  hasCoasterData: boolean;
 }
 
 const MONTHS = [
@@ -31,10 +33,16 @@ function shiftWeek(weekStart: string, delta: number): string {
   return d.toISOString().slice(0, 10);
 }
 
-export function WeekNav({ weekStart, weekDates, isCurrentWeek }: WeekNavProps) {
+export function WeekNav({ weekStart, weekDates, isCurrentWeek, coastersOnly, hasCoasterData }: WeekNavProps) {
   const router = useRouter();
-  const nav = (delta: number) =>
-    router.push(`/?week=${shiftWeek(weekStart, delta)}`);
+  const weekParam = `week=${weekStart}`;
+  const nav = (delta: number) => {
+    const base = `/?week=${shiftWeek(weekStart, delta)}`;
+    router.push(coastersOnly ? `${base}&coasters=1` : base);
+  };
+  const toggleCoasters = () => {
+    router.push(coastersOnly ? `/?${weekParam}` : `/?${weekParam}&coasters=1`);
+  };
 
   useEffect(() => {
     const onKey = (e: KeyboardEvent) => {
@@ -91,6 +99,30 @@ export function WeekNav({ weekStart, weekDates, isCurrentWeek }: WeekNavProps) {
       >
         →
       </button>
+
+      {hasCoasterData && (
+        <button
+          onClick={toggleCoasters}
+          style={{
+            marginLeft: 8,
+            display: "flex",
+            alignItems: "center",
+            gap: 5,
+            padding: "4px 12px",
+            borderRadius: 20,
+            border: coastersOnly ? "1px solid var(--color-accent)" : "1px solid var(--color-border)",
+            background: coastersOnly ? "var(--color-accent-muted)" : "var(--color-surface)",
+            color: coastersOnly ? "var(--color-accent)" : "var(--color-text-muted)",
+            fontSize: "0.72rem",
+            fontWeight: 600,
+            cursor: "pointer",
+            transition: "background 150ms ease, border-color 150ms ease, color 150ms ease",
+            whiteSpace: "nowrap",
+          }}
+        >
+          🎢 Coasters
+        </button>
+      )}
     </div>
   );
 }