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

package.json

@@ -10,8 +10,11 @@
     },
     "license": "MIT",
     "devDependencies": {
+        "@eslint/js": "^10.0.1",
         "@tailwindcss/vite": "^4.1.18",
         "autoprefixer": "^10.4.23",
+        "eslint": "^10.12.0",
+        "globals": "^17.13.0",
         "postcss": "^8.5.6",
         "prettier": "^3.8.1",
         "puppeteer-core": "25.5.0",