Error pages link a root-absolute favicon, which 404s on a sub-path site

[mmcky]

On a site served under a base URL, an error page's icon link still points at the domain root and 404s there. Normal pages are fine as of #227, which emits the favicon from the local Document where BASE_URL is available — but error pages do not render through that component.

What I verified in this repository:

Fact	Where
The root ErrorBoundary is upstream's component, re-exported as-is	app/root.tsx:27 — export { AppErrorBoundary as ErrorBoundary } from '@myst-theme/site'
The root route's links() supplies a root-absolute icon	app/root.tsx — { rel: 'icon', href: '/favicon.ico' }
That links() is deliberately where error-page head links live	the comment beside it: root's links() are the only ones that also apply when the root ErrorBoundary renders
links() cannot be made base-aware	it takes no arguments in Remix 1.17, and BASE_URL reaches the app only through the root loader

So the root-absolute href is not an oversight in links() — it is the only thing links() can express, and the base-aware version can only come from a component that receives BASE_URL. Upstream's AppErrorBoundary renders upstream's own Document, which this theme does not pass a base URL to.

I have not inspected the upstream package's internals to confirm how its Document builds head links; the conclusion above rests on this repository's own code and comments. Anyone picking this up should start there.

Scope

Only affects sites served under a sub-path (BASE_URL set) — which is the deployed lecture-wasm shape, and the PR-preview shape. A root-hosted site is unaffected, since the root-absolute path is already correct there. The visible symptom is a missing tab icon on 404 and error pages, so it is cosmetic rather than functional.

Possible directions

1. Wrap or replace the root ErrorBoundary with a local component that renders the theme's own Document, so the base-aware head links apply to error pages too. Keeps the fix in this repository.
2. Raise it upstream, so AppErrorBoundary's document can take a base URL. Wider benefit, but outside this repository's control.

Raised while reviewing #227, whose author flagged it as out of scope for that PR and correct to leave — the acceptance criterion there was only that error pages declare an icon link, which they do.

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