scripts/smoke.ts <url> checks /, /board, /healthz and the stylesheet as a phone and as a desktop

Replaces part of #2282. Both earlier tries went red only on unrelated suites (test/resilience.test.ts and test/tui-newforge.test.ts).

Done looks like:

[ ] bun scripts/smoke.ts <url> makes each request twice, once with an iPhone Safari user agent and once with a desktop Chrome user agent. /, /board (signed in with FORGE_TOKEN; a 302 to sign-in fails) and /healthz each answer 200. The stylesheet linked from / is a 200 text/css response with a non-empty body.
[ ] A failure exits non-zero and prints the URL, the user agent, the status and the content type. The script never shells out.
[ ] Test test/smoke.test.ts (listed in GATE) runs the script against a stub on a loopback port. It passes against a good stub and fails when the stub serves the stylesheet as text/html.
[ ] The pull touches only scripts/smoke.ts, the test and the GATE line. If an unrelated suite goes red, name it in the pull and change nothing else.

scripts/smoke.ts <url> checks /, /board, /healthz and the stylesheet as a phone and as a desktop #2339