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

test/viewport/harness.js

@@ -16,6 +16,10 @@ import { collectLayoutFacts } from "./facts.js";
 import { evaluateChecks, INTERACTIVE_SELECTORS } from "./checks.js";
 import { deriveViewports } from "./viewports.js";
 
+// page.waitForFunction and page.evaluate run the functions given them in
+// the page, where these are defined.
+/* global document, requestAnimationFrame */
+
 function required(name) {
     const value = process.env[name];
     if (!value) {
@@ -62,7 +66,6 @@ function stableHash(text) {
 
 async function connectBrowser() {
     const deadline = Date.now() + BROWSER_TIMEOUT_MS;
-    let lastError;
     for (;;) {
         try {
             const response = await fetch(`${CDP_URL}/json/version`);
@@ -77,9 +80,10 @@ async function connectBrowser() {
             });
             return { browser, version: info.Browser };
         } catch (error) {
-            lastError = error;
             if (Date.now() > deadline) {
-                throw new Error(`browser never came up: ${lastError}`);
+                throw new Error(`browser never came up: ${error}`, {
+                    cause: error,
+                });
             }
             await sleep(250);
         }