fix: resolve gateway URL at runtime via /api/studio fallback (#66)

Fixes #57 — NEXT_PUBLIC_GATEWAY_URL is a build-time variable that gets
baked into the client bundle. Changing it in .env and restarting has no
effect without a rebuild.

- normalizeLocalGatewayDefaults now accepts the sanitized public form
  ({url, tokenConfigured}) from /api/studio
- When no saved gateway URL exists, prefer runtime localGatewayDefaults
  (from openclaw.json or CLAW3D_GATEWAY_URL env var) over the
  potentially stale build-time NEXT_PUBLIC_GATEWAY_URL
- loadLocalGatewayDefaults falls back to CLAW3D_GATEWAY_URL/TOKEN env
  vars when openclaw.json is absent
- Added runtime env vars documentation to .env.example and README

Co-authored-by: robotica4us-collab <neo@openclaw.ai>
Made-with: Cursor

README.md

@@ -157,7 +157,8 @@ Common environment variables:
 
 - `HOST` and `PORT` control the Studio server bind address and port.
 - `STUDIO_ACCESS_TOKEN` protects Studio when binding to a public host.
-- `NEXT_PUBLIC_GATEWAY_URL` provides the default upstream gateway URL when Studio settings are empty.
+- `NEXT_PUBLIC_GATEWAY_URL` provides the default upstream gateway URL when Studio settings are empty. **Note:** this is a build-time variable — changes require `npm run build` to take effect.
+- `CLAW3D_GATEWAY_URL` and `CLAW3D_GATEWAY_TOKEN` provide a runtime alternative to `NEXT_PUBLIC_GATEWAY_URL` that takes effect on server restart without a rebuild. These are also used as a fallback when `openclaw.json` is not present.
 - `OPENCLAW_STATE_DIR` and `OPENCLAW_CONFIG_PATH` override the default OpenClaw paths.
 - `OPENCLAW_GATEWAY_SSH_TARGET`, `OPENCLAW_GATEWAY_SSH_USER`, `OPENCLAW_GATEWAY_SSH_PORT`, and `OPENCLAW_GATEWAY_SSH_STRICT_HOST_KEY_CHECKING` support advanced gateway-host operations over SSH when needed.
 - `ELEVENLABS_API_KEY`, `ELEVENLABS_VOICE_ID`, and `ELEVENLABS_MODEL_ID` enable voice reply integration.