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

src/lib/studio/settings-store.ts

@@ -44,8 +44,15 @@ const readOpenclawGatewayDefaults = (): { url: string; token: string } | null =>
   }
 };
 
-export const loadLocalGatewayDefaults = () => {
-  return readOpenclawGatewayDefaults();
+export const loadLocalGatewayDefaults = (): { url: string; token: string } | null => {
+  const fromFile = readOpenclawGatewayDefaults();
+  if (fromFile) return fromFile;
+  // Fall back to env vars so operators can configure the gateway URL at
+  // runtime without openclaw.json and without a rebuild.
+  const envUrl = process.env.CLAW3D_GATEWAY_URL?.trim();
+  const envToken = process.env.CLAW3D_GATEWAY_TOKEN?.trim();
+  if (envUrl) return { url: envUrl, token: envToken ?? "" };
+  return null;
 };
 
 export const loadStudioSettings = (): StudioSettings => {