chore: adopt ESLint flat config and add Builder smoke test

[nova-rey]

Summary

• replace the legacy ESLint configuration with a flat eslint.config.js that wires in TypeScript, React, and React Hooks rules while deferring formatting to Prettier
• add repository-wide Prettier configuration, scripts, and ignore file, reformatting existing sources to match the new style
• wire Vitest into Vite with a jsdom environment, add shared testing setup, and create a Builder render smoke test
• document the new checkpoint status across project docs and metadata files

Testing

• npm run format:check
• npm run lint
• npm run typecheck
• npm run test

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https://chatgpt.com/codex/tasks/task_e_6907c3babc8c8328bdb7ebd64bec5abf

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve base path in Vite config

The new vite.config.ts only adds Vitest configuration and drops the previous base: "./" (and related build options). Vite now defaults base to /, so the emitted bundle uses absolute asset URLs. The packaging workflow (scripts/build_ui_bundle.sh → ui_static.tar.gz described in the docs) depends on relative URLs so the static bundle can be served from an arbitrary sub-path or even opened directly from disk. After this change those deployments will 404 on all JS/CSS assets unless the bundle is hosted at the domain root. Consider keeping the original base/build options alongside the new test settings.

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