feat: HTML and WASM export of Reveal slides

[peter-gy]

Summary

A notebook configured as slides currently exports as a normal vertical notebook because its saved layout is not part of the exported mount data.

This PR carries the configured layout through static HTML and WebAssembly export, then restores it through the existing layout plugins before cells render. Static HTML opens as a Reveal deck with captured outputs. WebAssembly --mode run opens the same deck and keeps Python-backed controls reactive in the browser.

Both formats preserve slide types, fragments, speaker notes, deck settings, deep links, and code visibility. Ordinary notebooks retain their normal exported shell.

Implementation

• The CLI, server, in-app, and Pyodide export paths pass the same serialized layout through the existing mount configuration.
• The frontend holds that layout until notebook cells are available, then lets the selected renderer plugin validate and materialize it.
• In-app exports flush pending document changes before taking a snapshot. A full save recovers an ambiguous sync failure, and revision ordering prevents a slower auto-export from overwriting a newer file.
• Exported decks use the existing Reveal renderer for navigation, deep links, code controls, and notes.

Speaker view

Both formats embed speaker notes in the exported HTML, so anyone with the file can read them. Static HTML enables Reveal speaker view with S.

WebAssembly keeps speaker view disabled. Reveal creates the presenter previews by loading the deck twice in iframes, which would start two more Pyodide workers and rerun the notebook independently. Those previews could diverge from the main deck and repeat side effects. Clean support would need passive preview frames supplied by the main runtime, so this change keeps one Pyodide owner per deck. We can consider this as a follow up, but for now the effort-reward ratio seemed poor.

Closes #10682

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