fix(seer): navigate same-origin markdown links with the router

[natemoo-re]

The LLM sends fully resolved URLs like https://sentry.sentry.io/issues/ABC-123/ in its markdown responses. The <Markdown> component's DefaultLink treats these as external (they don't match the /^\/[^/]/ internal pattern), so every link renders as <ExternalLink> with target="_blank" — opening in a new tab even on plain click.

toRelativeHref strips the origin when the link points at the current app, so DefaultLink sees a relative path and uses the router <Link> instead.

[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

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^{Reviewed by Cursor Bugbot for commit 4a277c953dae2af813d75388c79658d1d8aaf1da. Configure here.}