add eslint & ci

[dale-wahl]

Adding automated linting and testing, mainly for the pull requests 4CAT opens automatically. Those carry map_item functions translated from Python by a language model, and nothing looked at them until a person did. Last batch arrived calling five helper functions that the same batch had deleted.

Continuous integration

• .github/workflows/ci.yml runs npm test and npm run lint on every push and pull request, drafts included
• The linter runs even when the tests fail, so one run reports both.
• The item comparer (npm run test:compare) stays out of CI; it needs a reachable 4CAT, an API key and dataset keys. (But we can look at add that next!)

Linting

• eslint.config.mjs at the repository root, running ESLint's recommended set of 58 rules. no-undef is the one that matters most here to catch those helpers called but never defined.
• Five style rules are off. The generated map_item bodies trip them, a sync replaces those blocks anyway, and none of them change what the code does.
• no-prototype-builtins is off too, and that one is not cosmetic: obj.hasOwnProperty(key) throws if a platform's JSON has a key by that name. 31 sites across js/ and modules/, worth its own PR.
• eslint and @eslint/js are pinned exactly because recommended is a moving list, so upgrading should be a decision rather than a side effect.
• tests/ and popup/ are excluded, with the reasons in the comments.

Housekeeping

• tests/Dockerfile.test deleted; I was using it but it stopped working and isn't needed
• engines added to tests/package.json: ESLint puts the Node floor at 20.19 so this made sense.
• eslint.config.mjs excluded from the release zip scripts
• One code change outside the tooling: a self-assignment removed from modules/gab.js (hand-written capture code, not a generated block), so no-self-assign can stay on.