Add repository formatting defaults

[Krisztiaan]

Summary

• add .editorconfig/.gitattributes to normalize whitespace and line endings across contributors
• document contributor setup with direnv support and optional tooling
• add .envrc plus ignore rules for per-project environments
• introduce a lint workflow that reports formatting issues without blocking builds (skips forked PRs)
• add ESLint dev dependency for the webapp so linting can run in CI

Rationale

These repo defaults prevent cross-platform diff churn and give new contributors a clear, reproducible environment setup while surfacing formatting drifts early.

Note

The actual code changes are pretty small; the bulk of the diff is formatting. To avoid repeated conflicts, you can cherry-pick the formatting commit into parallel branches. Also, the new lint workflow requires running under the upstream repository (or a maintainer rerun) to publish annotations, because forks lack checks:write permission.

[Krisztiaan]

Do not merge yet, GitHub check action coming up, and up for discussion! @grahamsz