Canonical host via APP_URL: stop login dumping users on /error from a host mismatch

The from-scratch dev login was broken: open the banner's http://localhost:3000, sign
in as the seeded admin, and you landed on http://127.0.0.1:3000/error "Page not found".
Root cause: the banner/APP_URL said localhost but kratos.yml hard-coded 127.0.0.1, and a
host-scoped Kratos CSRF cookie can't cross localhost<->127.0.0.1, so the cross-host login
POST lost it; Kratos redirected to its error sink, which the app had no route for (404).

Make APP_URL the single source of truth for the public host:
- Canonical-host redirect (app.ts): when APP_URL is set, an off-host GET/HEAD visitor is
  308'd to it (path+query kept) before a flow starts, so the browser, the themed forms and
  the cross-origin Kratos POST share one cookie host. After /public/ so static/health
  checks stay host-agnostic; GET/HEAD only so a 308 never replays a cross-host POST.
- Opt-in (no NODE_ENV / no magic default): unset => no redirect, so a prod deploy that
  forgets APP_URL can't bounce real users to a stale default. The dev stack sets it.
- kratos.yml browser URLs default to localhost (match APP_URL's dev value) and derive from
  ${APP_URL} via compose.override.yml; SERVE_PUBLIC_BASE_URL keeps the dev Ory port.
- Real /error page (views/error.ejs) replaces the catch-all 404 for genuine flow errors.

Tests-first: config (opt-in/validated), app (308 on mismatch, no-redirect on match, static
host-agnostic, POST untouched, /error page), updated kratos.test host pins. New devstack
regression (e2e/devstack-login.spec + compose.e2e-devstack.yml) drives the plain
docker-compose-up topology on the host network: login from localhost works and 127.0.0.1 is
canonicalised; wired into scripts/ci.sh. typecheck + 360 units + full ci.sh (visual 10 ·
auth 1 · oauth 2 · full 7 · devstack 2) green.

src/server.ts

@@ -39,6 +39,9 @@ log.info("plugins discovered", { count: plugins.length, ids: plugins.map((p) =>
 await runBootHooks(plugins); // plugin onBoot — after discovery, before listen; a throw aborts boot
 
 const server = createApp({
+  // Canonical-host redirect target (off-host GET/HEAD visitors are sent here). Opt-in: omitted unless
+  // APP_URL is set, so the redirect is fully off — and costs nothing — when unconfigured.
+  ...(config.appUrl ? { appUrl: config.appUrl } : {}),
   auth: { audience: config.jwtAudience, clockSkewSec: config.jwtClockSkewSec, issuer: config.jwtIssuer },
   cache: config.cacheTemplates,
   csrfSecret: config.csrfSecret,
@@ -53,7 +56,7 @@ const server = createApp({
   plugins,
   secureCookies: config.secureCookies,
 }).listen(config.port, () => {
-  log.info("listening", { port: config.port, url: `http://localhost:${config.port}` });
+  log.info("listening", { port: config.port, url: config.appUrl ?? `http://localhost:${config.port}` });
 });
 
 // Drain in-flight requests on container stop instead of cutting them mid-response, then flush any