fix(web): render HTML files in file preview

[tastelessjolt]

Opening an HTML workspace file currently shows source only, and served-web users cannot fall back to the Electron-only preview browser.

This adds the existing Source/Rendered toggle pattern to .html and .htm files. Rendered mode uses an iframe srcdoc sandbox that permits scripts while withholding same-origin access, forms, popups, top-level navigation, and downloads. Source remains the default, and line-reveal requests continue to select source mode.

Closes #5100

Screenshots

Before	After

Verification

• vp test run apps/web/src/components/files/FilePreviewPanel.test.ts
• vp run --filter @t3tools/web typecheck
• targeted lint and formatting checks
• served-web verification with styled .html and interactive .htm fixtures
• verified source switching, sandbox isolation, and in-frame script interaction

Built with GPT-5.6 Codex through the Codex harness in T3 Code.

Note

Render HTML files in the file preview panel with sandboxed iframe

• Adds isHtmlPreviewFile to detect .html/.htm files and prepareHtmlPreviewDocument to rewrite the document's <base href> to the signed asset directory URL, enabling relative assets to resolve correctly in preview.
• Adds a RenderedHtmlSurface component that fetches the signed asset URL, prepares the HTML document, and renders it in a sandboxed iframe with referrerPolicy=no-referrer.
• Integrates HTML rendering into FilePreviewPanel with a localStorage-backed toggle (t3code.renderHtml), sharing toggle UX with the existing Markdown render preference.
• Behavioral Change: .html/.htm files now default to a rendered preview (iframe) rather than raw source; users can toggle back to source view.

^{Macroscope summarized a63d845.}

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Note

Medium Risk
Sandboxed iframe preview runs authored scripts with allow-scripts, which is intentional but still a security-sensitive surface; base URL rewriting touches served asset paths.

Overview
Workspace .html / .htm files can now switch between source and rendered preview, mirroring the existing markdown toggle. The preference is stored in t3code.renderHtml; source stays the default, and line-reveal still forces source view.

Rendered HTML is shown in a sandboxed iframe (allow-scripts, no-referrer) via srcDoc. prepareHtmlPreviewDocument injects or rewrites a <base href> so relative assets resolve against the signed workspace asset URL.

Unit tests cover HTML file detection and base-tag preparation.

^{Reviewed by Cursor Bugbot for commit a63d845. Bugbot is set up for automated code reviews on this repo. Configure here.}

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[macroscopeapp]

Approvability

Verdict: Needs human review

This PR introduces a new feature that renders HTML files with JavaScript execution enabled in a sandboxed iframe. The security implications of executing user-provided scripts, even in a sandbox, warrant human review.

^{You can customize Macroscope's approvability policy. Learn more.}

[t3dotgg]

Note

🤖 GPT-6 Astra (preview) responding on behalf of Theo

This was closed as part of an automated cleanup pass. If you believe it was closed in error, reply here and we will get it reopened.

Closing as superseded by #9143, whose implementation is on main. The file panel now renders HTML with signed asset URLs and a source toggle, including sibling assets inside the workspace. The separate srcDoc renderer and base-tag rewriting in this PR are no longer needed.