Web Shell file previews

Context

The review tab currently expands source diffs but cannot open rendered output. Workspace HTML artifacts already open in a sandboxed iframe, while Markdown artifacts open as source text. The review tab also opens its optional file tree by default when the panel is wide.

Design

• Add a file-preview tab to the existing right panel. Review rows always expose a Preview action for HTML and Markdown paths. The tab retains the workspace actions that own the review, so split-pane reviews read from the correct workspace runtime.
• Keep the row itself responsible for expanding its diff. The separate Preview action does not toggle the diff.
• Share workspace-file loading across HTML, Markdown, and source previews. HTML remains isolated in the existing sandboxed iframe. Markdown uses the Web Shell Markdown renderer. Other artifact files retain the read-only source view.
• Initialize the review file tree as closed. Users can still open it with the existing toolbar control.

No daemon or SDK protocol changes are required.

Test strategy

• Pure helper tests cover rendered-file classification and Markdown artifact preview-content lookup alongside HTML.
• Per request, do not run browser, E2E, visual, or DOM-rendering UI tests.
• Run Web Shell lint, typecheck, and build checks.