Add repository diff viewer with structured file-level patches

## Summary
Upgrade repo detail pages from raw git show output to a cleaner diff UI.

## Scope
- Parse commit diffs into file-level sections.
- Show added, removed, and modified files clearly.
- Improve the repository detail experience on mobile.

## Acceptance Criteria
- Repo detail pages show readable per-file diffs.
- Large diffs degrade gracefully.
- Existing commit history remains available.
