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/lint

@@ -1,19 +1,21 @@
 #!/bin/sh
-# script/lint: lint the whole repo: prettier over the frontend, then the
-# Go linter over backend/.
+# script/lint: lint the whole repo: eslint over the frontend, then the Go
+# linter over backend/.
 #
-# The Go linter runs only in Docker: this builds the lint stage of
-# Dockerfile, the digest-pinned golangci-lint image, which runs the
-# backend's fmt-check and lint targets. --no-cache makes the linter
-# really run every time rather than reuse an earlier result, and the
-# stage is built for its checks alone, so no image is kept.
+# No linter runs on the host: this builds the frontend-lint and lint
+# stages of Dockerfile, the digest-pinned node and golangci-lint images.
+# The first runs eslint; the second runs the backend's fmt-check and
+# lint targets. --no-cache makes each linter really run every time
+# rather than reuse an earlier result, and each stage is built for its
+# checks alone, so no image is kept.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    "$ROOT/script/frontend-lint"
+    timeout 300 docker build --no-cache --target frontend-lint \
+        --output type=cacheonly .
     timeout 300 docker build --no-cache --target lint \
         --output type=cacheonly .
 }