Implement API key validation in app.js

Add API key check and secure startup message

Author: Mahadev-05

app.js

@@ -0,0 +1,6 @@
+const apiKey = process.env.WEATHER_API_KEY;
+if (!apiKey) {
+  console.error("CRITICAL ERROR: No API Key found!");
+  process.exit(1);
+}
+console.log("App is running securely.");