cf-error-pages: guard the inlined design tokens and storage host set against drift

[zachdunn]

scripts/cf-error-pages/ (PR #656) carries two hand-maintained copies of facts that live elsewhere in the repo. Neither has a guard, and both fail silently — the pages keep deploying, they just stop being correct.

1. Design tokens. build.mjs inlines the :root palette from packages/ui/dist/uploads-ui.css:

const TOKENS = `
  --bg: #0a0a0b;
  --panel: #121214;
  --line: #232327;
  ...
`;

The inlining is deliberate and can't be removed — a page shown because the origin is unreachable cannot fetch a stylesheet from that origin. But if the DS palette shifts, the error pages keep rendering in the old colors until someone notices and reruns build + deploy.

2. The storage host set. pages.mjs hardcodes which hosts the custom error rule matches:

expression: '(http.host in {"storage.uploads.sh" "store.uploads.sh" "embed.uploads.sh"} and http.response.code eq 404)',

This has already gone wrong once. The first pass omitted store.uploads.sh — documented in the "Dual public hosts" table in docs/ops.md as a durable public URL, resolving objects identically — so dead links there still returned the generic 27 KB Cloudflare page until it was caught during PR review. A fourth public storage host added later would reintroduce exactly that gap, silently.

Suggested fix

A test in the root vitest project asserting both:

• every --<name>: <value> in TOKENS matches the corresponding declaration in packages/ui/dist/uploads-ui.css
• the host set in the rule expression matches the storage hosts documented in docs/ops.md (or, better, a single exported constant that both the rule and the docs table are generated from)

The second is the higher-value half — the token drift is cosmetic and self-evident once someone looks at a page, whereas a missing host is invisible until a user hits a dead link.

Notes

• The build is not currently wired into CI at all; it only runs when someone deploys. A test would be the first thing that exercises it on every PR.
• Rendered HTML is gitignored (dist/), so a golden-file snapshot test would need to build first — cheap, the build is pure string templating with no network or browser.

Context: docs/ops.md → "Cloudflare error pages".