feat: add BuildInfo component to display build time and update Docker workflow to pass build time as environment variable

src/app/admin/page.tsx

@@ -1,6 +1,7 @@
 import DatasetsList from "@/components/DatasetsList";
 import CreateDatasetModal from "@/components/CreateDatasetModal";
 import { AdminTokenForm } from "@/components/AdminTokenForm";
+import BuildInfo from "@/components/BuildInfo";
 import { requireAdmin } from "@/lib/auth";
 import { redirect } from "next/navigation";
 import Link from "next/link";
@@ -30,6 +31,10 @@ export default async function AdminPage() {
         </div>
       </div>
 
+      <div className="mb-8">
+        <BuildInfo />
+      </div>
+
       <div className="mt-8">
         <h2 className="text-2xl font-bold mb-4">Datasets</h2>
         <CreateDatasetModal />

src/components/BuildInfo.tsx

@@ -0,0 +1,32 @@
+export default function BuildInfo() {
+  const buildTime = process.env.BUILD_TIME;
+
+  if (!buildTime) {
+    return null;
+  }
+
+  const buildDate = new Date(buildTime);
+  const now = new Date();
+  const diffMs = now.getTime() - buildDate.getTime();
+  
+  let timeAgo = '';
+  const diffMins = Math.floor(diffMs / 60000);
+  const diffHours = Math.floor(diffMs / 3600000);
+  const diffDays = Math.floor(diffMs / 86400000);
+
+  if (diffMins < 60) {
+    timeAgo = `${diffMins} minute${diffMins !== 1 ? 's' : ''} ago`;
+  } else if (diffHours < 24) {
+    timeAgo = `${diffHours} hour${diffHours !== 1 ? 's' : ''} ago`;
+  } else {
+    timeAgo = `${diffDays} day${diffDays !== 1 ? 's' : ''} ago`;
+  }
+
+  return (
+    <div className="bg-gray-50 border border-gray-200 rounded-lg p-4 text-sm">
+      <p className="text-gray-700">
+        <span className="font-semibold">Build Time:</span> {buildDate.toLocaleString()} ({timeAgo})
+      </p>
+    </div>
+  );
+}