Use ESLint Stylistic without hard wrapping

[samuel-williams-shopify]

Summary

• replace Prettier with ESLint Stylistic for fixable JavaScript style rules
• preserve authored line breaks by omitting line-width and reflow rules
• enforce tab indentation through ESLint and EditorConfig
• add a dedicated style check to CI

This is an alternative to #5. It enforces the existing preferences for tabs, single quotes, semicolons, spacing, and trailing commas without deciding when arrays, calls, or expressions should wrap.

Churn

The JavaScript changes are almost entirely removal of trailing whitespace, plus two import-spacing fixes and one quote normalization. Most of the raw diff size comes from adding ESLint dependencies to package-lock.json.

Validation

• npm ci
• npm run format:check
• npm test (858 tests)