fix(markdown): keep the preview-only scroll position across remounts (#1009)

Tab switches and mode toggles unmount MarkdownPreview, whose only
scroll state was the DOM scrollTop; Preview Only started over at the
top. Record the preview's own scrollTop in the per-path live markdown
store and restore it on mount (split mode keeps its editor-driven
sync).

src/ui/selectableSurfaces.test.tsx

@@ -22,7 +22,7 @@ describe("selectable read-only surfaces", () => {
   it("Markdown preview (the reported case)", () => {
     act(() =>
       useLiveMarkdown.setState({
-        "/p/README.md": { text: "# Title\n\nSome *text*", topLine: 0, scrollSeq: 0, fontZoom: 0 },
+        "/p/README.md": { text: "# Title\n\nSome *text*", topLine: 0, scrollSeq: 0, fontZoom: 0, previewTop: 0 },
       }),
     );
     act(() => useMarkdownModes.setState((s) => ({ byPath: { ...s.byPath, "/p/README.md": "preview" } })));