Lint static/js/ with ESLint in Docker (closes #120)

ESLint, pinned by package.json and yarn.lock, runs in a new js-lint
stage of the Dockerfile on the pinned node 24 LTS image. script/lint
builds that stage after the Go lint, and the build stage depends on it,
so make check and the image build both fail on a violation.
eslint.config.mjs turns on the styleguide's checkable rules: no-var and
prefer-const.

Model: opus-5-5

package.json

@@ -0,0 +1,6 @@
+{
+    "private": true,
+    "devDependencies": {
+        "eslint": "10.11.0"
+    }
+}