Lint the frontend with eslint in its own Docker stage (closes #47)

script/lint ran prettier --check, the same check script/fmt-check
runs, so the JavaScript had no linter. eslint now runs with its
recommended rules, set in eslint.config.js, in a new frontend-lint
stage of Dockerfile built from the pinned node image and the lockfile.
The frontend stage copies a file from it, as the builder stage does
from the Go lint stage, so the image cannot build unless eslint passed.
script/lint builds both lint stages with --no-cache and runs no linter
on the host; script/fmt-check keeps prettier on the host, and
script/frontend-check drops its lint step. The viewport harness fixes
the two kinds of finding eslint made. bootstrap wants node 22.13.0, as
eslint 10 does. Also covers item 2 of
#28.

Model: opus-5-5

README.md

@@ -42,25 +42,28 @@ halves, so the root `make check` fails if either one is broken. We provide:
   as new as `backend/go.mod` asks for is installed, the Go modules, and gcc with
   the C library headers unless gcc is installed, for the race detector in
   `make test`), linking what it installs itself into `~/.local/bin`, which has
-  to be on `PATH`. It installs no Go linter and not Docker: `make lint` runs the
-  linter in Docker
+  to be on `PATH`. It installs no Go linter and not Docker: `make lint` runs
+  both linters in Docker
 - `script/setup` — make a fresh clone ready for development: bootstrap plus the
   git pre-commit hook
 - `script/projectname` — print the project name (used for the Docker image tag)
 - `script/test` — run `script/frontend-test`, then the backend's Go tests, each
   under its own 30-second timeout
-- `script/lint` — run `script/frontend-lint`, then golangci-lint in Docker, by
-  building the lint stage of `Dockerfile` without the cache
+- `script/lint` — run eslint, then golangci-lint, both in Docker, by building
+  the `frontend-lint` and `lint` stages of `Dockerfile` without the cache
 - `script/fmt` — format all files (writes): prettier, then gofmt over `backend/`
 - `script/fmt-check` — check formatting (read-only): prettier, then gofmt
 - `script/check` — run test, lint, and fmt-check
 - `script/frontend-test` — run the unit tests in `test/unit/` with Node's
   built-in test runner, then the production build
-- `script/frontend-lint` — run prettier in check mode
+- `script/frontend-lint` — run eslint with the rules in `eslint.config.js`; it
+  runs inside the `frontend-lint` stage of `Dockerfile`, which `make lint`
+  builds
 - `script/frontend-fmt` — format everything prettier understands (writes)
 - `script/frontend-fmt-check` — check prettier formatting (read-only)
-- `script/frontend-check` — the frontend half of `script/check`, for
-  `Dockerfile`, whose node build stage has neither Go nor Docker
+- `script/frontend-check` — run `script/frontend-test` and
+  `script/frontend-fmt-check`, for the frontend stage of `Dockerfile`, which has
+  neither Go nor Docker
 - `script/frontend-viewport-test` — responsive-layout verification of the built
   frontend in a containerised headless Chrome (see
   [test/viewport/README.md](test/viewport/README.md)). Not part of
@@ -265,7 +268,6 @@ properties.
 ## TODO
 
 - Add unit tests
-- Add eslint for JS linting (currently lint target runs prettier only)
 - Add configurable host list (environment variable or config file)
 - Add latency history export (CSV/JSON)
 - Add notification/alert when status changes to DEGRADED