fix: preserve stylesheet base URL in shadow roots

[cdeil]

This pull request was authored by a coding agent.

📝 Summary

When copyStyles serializes a linked stylesheet into a constructed stylesheet for a component shadow root, WebKit resolves relative font URLs against the document directory instead of the source stylesheet. Nested static exports therefore request bundled fonts from the lesson root and receive 404 responses.

This change:

• constructs each copied stylesheet with baseURL: sheet.href ?? document.baseURI
• covers both linked and inline stylesheets through the real registerReactComponent path

CSSStyleSheetInit.baseURL is the CSSOM mechanism for setting the base URL used to resolve relative URLs.

🔬 Verification

• Red: the two new regression cases fail on unchanged current main; the 14 existing focused tests pass.
• Green: all 16 focused tests pass after the production fix.
• Full make check, frontend lint, type-check, and production build pass.
• Nested html-wasm export:
  • Safari 26.6.2: 9 wrong-base font requests before, 0 after
  • Playwright WebKit 26.5: 9 before, 0 after
  • Chromium 151: 0 before and after

📋 Pre-Review Checklist

• For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on Discord, or the community discussions (Please provide a link if applicable).
• Any AI generated code has been reviewed line-by-line by the human PR author, who stands by it.
• Video or media evidence is provided for any visual changes (optional).

✅ Merge Checklist

• I have read the contributor guidelines.
• Documentation has been updated where applicable, including docstrings for API changes.
• Tests have been added for the changes made.

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Project	Deployment	Actions	Updated
marimo-docs	Ready	Preview	Sep 3, 2026 7:10pm UTC

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[cdeil]

I have read the CLA Document and I hereby sign the CLA