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

Makefile

@@ -30,8 +30,8 @@ fmt-check:
 check:
 	@script/check
 
-# The frontend half of check, for Dockerfile's node build stage, which
-# has neither Go nor Docker. Use check everywhere else.
+# The frontend tests and format check, for Dockerfile's frontend stage,
+# which has neither Go nor Docker. Use check everywhere else.
 frontend-check:
 	@script/frontend-check