Add ESLint and Prettier

Prettier owns formatting: .prettierrc.json, format and format:check scripts, and
a one-off pass over the repo so the check is green from the start. Embedded
language formatting is off deliberately - the renderers build HTML in html
tagged templates and letting Prettier reflow those would change the markup
customers receive. docs/ is excluded because it is hand-wrapped prose, and shell
scripts have no Prettier parser.

ESLint covers the JavaScript: the two browser scripts, the build and check
scripts, and its own config. no-var is scoped out for the browser scripts on
purpose; they are plain ES5-shaped IIFEs and a hoisting difference would only
show up in a browser.

ESLint cannot lint the TypeScript here: typescript@7 is the native port with no
JavaScript compiler API, and typescript-eslint requires a 4.8-6.1 peer, so it
refuses to install and forcing it through would leave a parser that cannot read
the compiler. tsc --noEmit stays the TypeScript gate; README records the two
ways to change that.

lint and format:check now run in deploy.sh, and VS Code formats on save with
Prettier and surfaces ESLint findings.

scripts/check-site.ts

@@ -62,7 +62,8 @@ record('GET /definitely-not-here', missingPage.status === 404, `status ${missing
 
 // --- Each Section a customer can reach, and its images -----------------------
 
-const sections = landing === null ? [] : [...landing.matchAll(/class="row" href="([^"]+)"/g)].map((m) => m[1]!);
+const sections =
+  landing === null ? [] : [...landing.matchAll(/class="row" href="([^"]+)"/g)].map((m) => m[1]!);
 
 if (sections.length === 0) {
   record('at least one Section has a Current Menu Set', false, 'the landing page lists none');
@@ -121,5 +122,7 @@ for (const result of results) {
 }
 
 const failures = results.filter((result) => !result.ok);
-console.log(`\n${results.length - failures.length}/${results.length} checks passed against ${base}`);
+console.log(
+  `\n${results.length - failures.length}/${results.length} checks passed against ${base}`,
+);
 if (failures.length > 0) process.exitCode = 1;