Document WS_ENDPOINT in .env.sample for local dev

.env.sample

@@ -1,6 +1,16 @@
 # SEIZE API ENDPOINT - for production, use 'https://api.6529.io'
 API_ENDPOINT=
 
+# REALTIME WEBSOCKET ENDPOINT (optional) - for production, use
+# 'wss://ws.6529.io'. Set this in local development to your backend's ws port
+# (e.g. 'ws://localhost:3000'). When unset, the app derives the URL from
+# API_ENDPOINT, but that only rewrites the production 'https://api...' host to
+# 'wss://ws...'; any other API_ENDPOINT (such as a localhost URL) is passed to
+# the WebSocket constructor unchanged and fails, so live wave updates don't
+# work locally without setting this. Keep the line commented out when unused:
+# an empty value fails env validation ('WS_ENDPOINT must be a valid URL').
+# WS_ENDPOINT=
+
 # ALLOWLIST API ENDPOINT - for production, use 'https://allowlist-api.6529.io'
 ALLOWLIST_API_ENDPOINT=