Fix external themes in serve mode

[panglesd]

We need to resolve the theme before it being delayed. The un-delaying is done in the browser client, which does not have access to file system.

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR refactors how external themes are represented and resolved. The theme type in frontmatter is changed from storing external themes as raw strings to storing them as Asset.t with location information. The theme parsing logic is updated to convert non-builtin theme strings into assets during frontmatter parsing. Correspondingly, the delayed_from_units function signature is simplified by removing the read_file callback parameter, since theme resolution now happens at parse time rather than at render time. The CLI serve function and related call sites are updated to match the new API.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: fixing external themes in serve mode, which is the core issue addressed throughout the changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The PR description clearly explains the fix: themes must be resolved server-side before being delayed, as the browser client lacks file system access.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands and usage tips.}