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/config.test.ts

@@ -28,6 +28,14 @@ test("loads dev defaults when the environment is empty", () => {
   assert.equal(c.otlpEndpoint, undefined); // OTLP export opt-in; console-only by default
   assert.equal(c.otlpProtocol, "http/json");
   assert.equal(c.serviceName, "plainpages"); // OTLP service.name default; implementer-overridable
+  assert.equal(c.appUrl, undefined); // canonical-host redirect is an explicit opt-in (off unless APP_URL set)
+});
+
+test("APP_URL is the canonical public URL: opt-in (unset ⇒ no redirect), honoured when set, validated", () => {
+  assert.equal(loadConfig({}).appUrl, undefined); // unset ⇒ off, so a prod default can't misfire
+  assert.equal(loadConfig({ APP_URL: "" }).appUrl, undefined); // empty ⇒ off too (compose passes ${APP_URL:-})
+  assert.equal(loadConfig({ APP_URL: "https://admin.acme.com" }).appUrl, "https://admin.acme.com");
+  assert.throws(() => loadConfig({ APP_URL: "not a url" }), /APP_URL/);
 });
 
 test("SERVICE_NAME is overridable so an implementer brands their own logs/traces (§9)", () => {