docs: add ALLOWED_ORIGINS to .env.example

.env.example

@@ -9,3 +9,7 @@ XIMILAR_API_KEY=your-ximilar-api-key-here
 # Perplexity API Key (AI deck research & generation)
 # Get yours at: https://www.perplexity.ai/settings/api
 PPLX_API_KEY=your-perplexity-api-key-here
+
+# CORS allowed origins (comma-separated)
+# Default: http://localhost:5173,http://localhost:8080
+ALLOWED_ORIGINS=http://localhost:5173,http://localhost:8080