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

eslint.config.js

@@ -0,0 +1,27 @@
+// eslint's recommended rules over every JavaScript file in the repo.
+// script/frontend-lint runs it, in the frontend-lint stage of Dockerfile.
+import js from "@eslint/js";
+import globals from "globals";
+import { defineConfig } from "eslint/config";
+
+export default defineConfig([
+    js.configs.recommended,
+    {
+        // The page, and facts.js, which the viewport test runs inside it.
+        files: ["src/**/*.js", "test/viewport/facts.js"],
+        languageOptions: {
+            globals: {
+                ...globals.browser,
+                // vite.config.js defines these when it builds the page.
+                __COMMIT_HASH__: "readonly",
+                __COMMIT_FULL__: "readonly",
+            },
+        },
+    },
+    {
+        // Run by node: the tests, the viewport harness and these configs.
+        files: ["test/**/*.js", "*.js"],
+        ignores: ["test/viewport/facts.js"],
+        languageOptions: { globals: globals.node },
+    },
+]);