feat(config): add environment variable validation

Author: maitamdev

src/utils/envValidation.ts

@@ -0,0 +1,7 @@
+// Environment variable validation
+export function validateEnv(): void {
+  const required = ['VITE_FIREBASE_API_KEY', 'VITE_FIREBASE_AUTH_DOMAIN', 'VITE_FIREBASE_PROJECT_ID'];
+  const missing = required.filter(key => !import.meta.env[key]);
+  if (missing.length > 0) { console.warn('Missing env vars:', missing.join(', ')); }
+}
+