fix: improve admin security and modal accessibility

- Replace Card-based modal with proper Dialog component in SecurityAlertsTable for better accessibility
- Add missing admin role check to threat-analysis endpoint for proper authorization
- Implement ARIA attributes, focus management, and semantic structure
- Ensure consistent admin security patterns across endpoints

app/api/admin/security-monitoring/threat-analysis/route.ts

@@ -29,7 +29,7 @@ export async function POST(request: NextRequest) {
   try {
     const session = await getServerSession(authOptions);
 
-    if (!session?.user || !session.user.isPlatformUser) {
+    if (!session?.user || session.user.role !== "ADMIN") {
       return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
     }