feat(deploy): configure backend URL at runtime

[DIvanCode]

Summary

• build the Frontend image with a runtime-replaceable backend URL marker
• inject VITE_BASE_URL into the generated JavaScript when the container starts
• move VITE_BASE_URL from the image build playbook to the deployment container environment

Why

Vite previously embedded VITE_BASE_URL while building the image, so changing the backend required rebuilding the Frontend image. The same image can now be started with different backend URLs.

Verification

Not run by explicit request: local tests, build, and production-bundle browser smoke test were waived for this publication.

What to test manually

Prerequisite: a reachable backend and, for authenticated checks, a test account.

1. Deploy the image with VITE_BASE_URL set to the backend API URL.
2. Open the application and confirm the authentication screen loads and API requests use that URL.
3. Sign in and open a duel-capable screen; confirm HTTP requests and the user WebSocket connect through the configured backend.
4. Restart the same image with another backend URL and confirm the application switches targets without rebuilding the image.
5. As regressions, verify direct-route reloads still fall back to index.html and static assets are served normally.

Closes #92

[chatgpt-codex-connector]

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