Fix StatCard crash: handle string values from Postgres counts

formatNum now safely parses string numbers instead of
crashing on .toFixed() when Postgres returns count as string.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: a16i

web/src/components/StatCard.tsx

@@ -1,9 +1,11 @@
 import { Sparkline } from "./Sparkline";
 
-function formatNum(n: number): string {
-  if (n >= 1_000_000) return `${(n / 1_000_000).toFixed(1)}M`;
-  if (n >= 1_000) return `${(n / 1_000).toFixed(1)}K`;
-  return n.toString();
+function formatNum(n: number | string): string {
+  const v = typeof n === "string" ? parseFloat(n) : n;
+  if (isNaN(v)) return "0";
+  if (v >= 1_000_000) return `${(v / 1_000_000).toFixed(1)}M`;
+  if (v >= 1_000) return `${(v / 1_000).toFixed(1)}K`;
+  return v.toString();
 }
 
 export function StatCard({