Follow-up to the Render | Source toggle for Files-pane HTML: in source mode the pane rendered its own switcher bar above LocalFilePreview's header, so the file showed two stacked header rows where Markdown shows one. LocalFilePreview now takes `onSelectRendered`; when present it adds `rendered` to its own switcher (next to Edit, as for Markdown) and routes the selection back to the pane. The pane-level switcher renders only in rendered mode, above the browser bar.
Also gate the toggle on a renderable target: a remote HTML file whose data URL failed validation arrives as `{ renderMode: 'source', transient: true }` with no `dataUrl`, and offering PREVIEW there would load a `file://` URL of a remote path.
Test asserts PREVIEW and Edit share one header row in source mode, and that the transient source fallback offers no PREVIEW. Prettier on the two lines the base commit left unformatted.
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