feat: preview verified local files and directories from chat

[wyx20021207-create]

Summary

• Recognize local path references in Markdown text and inline code, without rewriting fenced code, math, or existing links.
• Validate candidates using the existing authorized file metadata API before making them clickable. Nonexistent/inaccessible paths retain their original text.
• Keep the original path label and neutral code-style background; show the resolved full path on hover and open it in the right-hand panel.
• Support directory metadata and a right-panel directory listing, with navigation into child directories and files.
• Recheck before opening, periodically refresh validation, deduplicate concurrent checks, and cap metadata request concurrency.
• Keep the existing allowed-root and realpath checks; do not broaden filesystem access.

Testing

• Markdown rendering, path validation, directory dispatch, and existing file-viewer regression tests pass.
• npm run lint and TypeScript tsc --noEmit pass in the development checkout.
• Ran npm test on Windows and compared it with an untouched upstream/main checkout at 0d1df12: both have the same 14 failing tests (including Windows symlink/shell limitations and existing component assertions). No additional failing tests were observed.
• Production build and browser E2E were not run locally.

Manual verification

1. Include an existing relative path or Windows absolute path in a chat response.
2. Verify the original label remains visible, the full path appears on hover, and clicking opens the right panel.
3. Include a nonexistent path or slash-separated function names and verify they remain plain text/code.
4. Click an existing directory and navigate through its entries in the right panel.

This PR is independent of the composer image-preview feature.