Fix fleet stale-status flicker during polling

src/lib/server-freshness.ts

@@ -0,0 +1,10 @@
+// Allow a polling cycle plus time for queued RCON requests and delivery.
+export const STATUS_REFRESH_MS = 30_000;
+export const ONLINE_MAX_MS = STATUS_REFRESH_MS + 15_000;
+// The UI clock ticks once per second and can trail a newly arrived report.
+const CLOCK_TOLERANCE_MS = 1_000;
+
+export function freshStatusTimestamp(timestamp: string | null, now: number): boolean {
+    const age = now - Date.parse(timestamp ?? '');
+    return age >= -CLOCK_TOLERANCE_MS && age <= ONLINE_MAX_MS;
+}