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

script/frontend-check

@@ -1,9 +1,10 @@
 #!/bin/sh
-# script/frontend-check: run the frontend half of the checks only (test,
-# lint, fmt-check). This exists for the frontend stage of Dockerfile, a
-# node image with neither Go nor Docker; the Dockerfile's lint and
-# backend build stages gate the backend half. Everywhere else, use
-# script/check, which covers the whole repo. Must not modify any files.
+# script/frontend-check: run the frontend tests and format check only.
+# This exists for the frontend stage of Dockerfile, a node image with
+# neither Go nor Docker; the Dockerfile's frontend-lint stage runs the
+# frontend linter, and its lint and builder stages gate the backend.
+# Everywhere else, use script/check, which covers the whole repo. Must
+# not modify any files.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -11,7 +12,6 @@ ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 main() {
     cd "$ROOT"
     "$ROOT/script/frontend-test"
-    "$ROOT/script/frontend-lint"
     "$ROOT/script/frontend-fmt-check"
 }