Add rendered markdown preview toggle to the file diff panel

[pufit]

What

In the session file-changes panel (right side panel), markdown files now get an eye toggle in the detail header that switches between the raw diff and a rendered markdown preview of the file.

How

Backend — compute_file_diff now attaches two fields to the diff response for .md/.markdown files (case-insensitive):

• markdown_content: the post-change file content; falls back to the original content for deleted files so the preview can still show what was removed. null for non-markdown files.
• markdown_truncated: true when the content was cut at MAX_DIFF_LINES (mirrors the patch truncation to keep payloads bounded).

Frontend — FileDetailView shows an Eye toggle (same pattern as the line-wrap toggle) only when markdown_content is present. Preview mode renders through the existing MarkdownContent component (GFM tables, syntax-highlighted code blocks). The toggle resets to diff view when switching files, and a truncation notice is shown when the preview was cut.

Old-backend / new-frontend degrades gracefully: no field → no toggle, diff panel unchanged.

Testing

• 5 new unit tests in tests/test_diff.py (md content present, case-insensitive suffix, deleted-file fallback, non-md null, truncation) — full suite 1370 passed
• npm run build clean
• Verified in the browser: toggle renders headings/bold/tables/code preview, switches back to raw diff, absent for non-markdown files

Generated by Nerve