refactor(store): remove @multica/fetch dependency

- Move gatewayUrl into gateway store state with setGatewayUrl action
- layout.tsx uses useGatewayStore.getState().setGatewayUrl() directly
- Remove @multica/fetch from store and web package.json dependencies

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/package.json

@@ -9,7 +9,6 @@
     "lint": "eslint"
   },
   "dependencies": {
-    "@multica/fetch": "workspace:*",
     "@multica/sdk": "workspace:*",
     "@multica/store": "workspace:*",
     "@multica/ui": "workspace:*",