perf(landing): share one SSE socket for online counters, respect reduced motion

Header and hero/stats counters each opened their own EventSource to the
online-count stream; a shared subscriber now opens a single socket and
multicasts to every mounted counter. The entrance count-up animation skips
its requestAnimationFrame loop when the user prefers reduced motion.

src/lib/online-count-sse.ts

@@ -0,0 +1,77 @@
+"use client";
+
+import { useEffect, useState } from "react";
+
+type Listener = (count: number) => void;
+
+/* Single shared EventSource for /api/online/count/stream. Every subscribing
+ * component re-uses one connection (multicast to all listeners) instead of
+ * opening its own SSE socket per component. */
+const MAX_RETRIES = 8;
+const BASE_DELAY_MS = 1_000;
+const MAX_DELAY_MS = 30_000;
+
+let eventSource: EventSource | null = null;
+const listeners = new Set<Listener>();
+let refCount = 0;
+let reconnectTimer: ReturnType<typeof setTimeout> | undefined;
+let attempts = 0;
+
+function start() {
+	if (eventSource || refCount === 0) return;
+	eventSource = new EventSource("/api/online/count/stream");
+	eventSource.onopen = () => {
+		attempts = 0;
+	};
+	eventSource.onmessage = (event) => {
+		try {
+			const parsed = JSON.parse(event.data) as { count?: number };
+			if (typeof parsed.count === "number") {
+				const count = parsed.count;
+				listeners.forEach((listener) => {
+					listener(count);
+				});
+			}
+		} catch {
+			// ignore malformed messages
+		}
+	};
+	eventSource.onerror = () => {
+		eventSource?.close();
+		eventSource = null;
+		if (refCount === 0 || attempts >= MAX_RETRIES) return;
+		const delay = Math.min(BASE_DELAY_MS * 2 ** attempts, MAX_DELAY_MS);
+		attempts++;
+		reconnectTimer = setTimeout(start, delay);
+	};
+}
+
+function stop() {
+	if (refCount > 0 || !eventSource) return;
+	if (reconnectTimer !== undefined) clearTimeout(reconnectTimer);
+	eventSource.close();
+	eventSource = null;
+	attempts = 0;
+}
+
+/**
+ * Live online-user count shared by the header and the hero/stats counters.
+ * Falls back to the server-rendered `initial` value until the stream reports.
+ */
+export function useOnlineCount(initial: number): number {
+	const [count, setCount] = useState(initial);
+
+	useEffect(() => {
+		const listener: Listener = setCount;
+		refCount++;
+		listeners.add(listener);
+		start();
+		return () => {
+			listeners.delete(listener);
+			refCount--;
+			stop();
+		};
+	}, []);
+
+	return count;
+}