Web had no way to see an HTML file as a page: the integrated browser is
desktop-only, so the file panel always showed markup. Mobile already renders
browser files in a WebView.

The panel now shows HTML and PDF as a rendered page from the signed asset
URL, with a source toggle for HTML that works like the Markdown one. HTML
runs in a sandboxed frame with an opaque origin so a page cannot reach the
app's session; workspace files keep loading sibling assets, host files are
served on their own.

Co-Authored-By: Claude Code <noreply@anthropic.com>