docs: Add placeholder for Clerk production key in .env.production

- Add TODO comment to replace with actual production Clerk key
- Production key should start with pk_live_ (not pk_test_)
- Current test key causes development key warnings in production

.env.production

@@ -1,4 +1,9 @@
 # Production Environment Variables
 # Port 3005 is proxied to https://litter-api.psbailey.uk/
 VITE_API_BASE_URL=https://litter-api.psbailey.uk/api
-VITE_CLERK_PUBLISHABLE_KEY=pk_live_Y2xlcmsubGl0dGVyc2NvdXRzLnBzYmFpbGV5LnVrJA
+
+# Clerk Production Key
+# Get your production publishable key from: https://dashboard.clerk.com
+# It should start with pk_live_ (not pk_test_)
+# TODO: Replace with actual production key from Clerk dashboard
+VITE_CLERK_PUBLISHABLE_KEY=pk_live_YOUR_PRODUCTION_KEY_HERE