fix: use-proxy-api-url-version-endpoint

[Zaimwa9]

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Changes

Closes #6588

Frontend was only handling api/v1/* when proxying traffic with FLAGSMITH_PROXY_API_URL while version lives at the root, causing the frontend to hit its own /version, ignoring is_enterprise and is_saas in deployments

• Add /version in proxied paths when FLAGSMITH_PROXY_API_URL is set
• Renamed FE version endpoint to /_frontend_version to avoid conflicts
• Update useBuildVersion to use /_frontend_version

How did you test this code?

• Start a flagsmith api via docker
• in frontend start the server FLAGSMITH_PROXY_API_URL=http://localhost:8000 ENV=local npm run dev
  Run:
  curl http://localhost:8080/version/ => response includes is_enterprise is_saas
  and
  curl http://localhost:8080/_frontend_version

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Project	Deployment	Actions	Updated (UTC)
flagsmith-frontend-preview	Ready	Preview, Comment	Feb 20, 2026 5:42pm
flagsmith-frontend-staging	Ready	Preview, Comment	Feb 20, 2026 5:42pm

1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
docs	Ignored		Feb 20, 2026 5:42pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-6743	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-6743	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-6743	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-6743	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6743	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-6743	Finished ✅	Results ✅