feat(manager-app): Part 5 — show snapshot_taken_at freshness in DashboardPage

Adds a "Site data: X min ago" line derived from snapshot.snapshot_taken_at
so managers can see how fresh the stats snapshot is.

All other Part 5 items (menu-app api.js, OrderConfirm polling, manager-app
api.js, IncomingOrdersPage 15s poll + OrderCard actions) were already fully
implemented in the existing scaffolding.

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

connect_frontend/manager-app/src/pages/DashboardPage.jsx

@@ -64,10 +64,20 @@ export default function DashboardPage() {
 
         {lastUpdated && (
           <p className="text-xs text-slate-400 text-right">
-            Updated {lastUpdated.toLocaleTimeString()}
+            Refreshed {lastUpdated.toLocaleTimeString()}
           </p>
         )}
 
+        {stats.snapshot_taken_at && (() => {
+          const diffMin = Math.round((Date.now() - new Date(stats.snapshot_taken_at).getTime()) / 60_000)
+          const label = diffMin < 1 ? 'just now' : `${diffMin} min ago`
+          return (
+            <p className="text-xs text-slate-400 text-right -mt-3">
+              Site data: {label}
+            </p>
+          )
+        })()}
+
         {/* Pending orders */}
         <div className="space-y-3">
           <div className="flex items-center justify-between">