[Bug]: Rendered-markdown view for a file in the right sidebar resets to source when you switch threads

[kristjaningi]

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/web

Steps to reproduce

1. Open a thread and open a .md file in the right-hand file preview panel.
2. Click the eye toggle ("Show rendered markdown"). The panel switches to the
   rendered view.
3. Switch to a different thread.
4. Switch back to the original thread and open the same .md file.

Expected behavior

The panel is still showing rendered markdown. The view mode should be sticky —
once I've said I want to read markdown rendered, that's my preference until I
toggle it back.

Actual behavior

The panel is back to raw markdown source, and I have to click the toggle again
every time I return to the thread.

Impact

Minor bug or occasional failure

Version or commit

main @ 5719e8a

Environment

macOS 26.5.2, desktop app

Logs or stack traces

The toggle is backed by component-local state in `FilePreviewPanel`:

  apps/web/src/components/files/FilePreviewPanel.tsx:675
    const [markdownView, setMarkdownView] = useState<{
      path: string | null;
      revealRequestId: number | null;
    }>({ path: null, revealRequestId: null });

Nothing persists it, and it's invalidated two ways:

- `renderMarkdown` requires `markdownView.path === relativePath`
  (FilePreviewPanel.tsx:681), so any change of the open file drops back to
  source.
- The panel is mounted with `key={environmentId:workspaceRoot}`
  (ChatView.tsx:5588), so switching to a thread in another project remounts it
  and resets the state entirely.

The adjacent explorer-open toggle in the same component already persists via
`FILE_EXPLORER_STORAGE_KEY` in localStorage (FilePreviewPanel.tsx:701-711),
which looks like the precedent to follow here.

Screenshots, recordings, or supporting files

No response

Workaround

Click the toggle again each time.