Allow configuring backend URLs at container startup

[yzxcj797]

Summary

• generate backend configuration when the Docker container starts instead of baking it into the Vite bundle
• use runtime backend URLs for gist and email requests while preserving build-time configuration as a fallback
• safely encode runtime values, prevent stale config caching, and document prebuilt-image usage

Fixes #1025

Testing

• npm test - 7 passed
• npm run lint - passed
• npm run build - passed (only existing dependency/chunk-size warnings)
• npx prettier --check ... on all changed supported files - passed
• bash -n docker/40-drawdb-runtime-config.sh - passed
• entrypoint execution test covers Unicode, quotes, newlines, </script>, and ampersands

Docker Desktop was unavailable locally, so the complete image was not started. The production entrypoint script itself was executed and its generated JavaScript was parsed in the regression test.

AI assistance

OpenAI Codex assisted with diagnosis, implementation, tests, and review. The final diff and behavior were reviewed and validated locally.

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