- Add a new /api/git/diff/file endpoint with path and staged query handling, including validation and untracked-file diff support
- Introduce fetchGitFileDiff API client wiring and update GitManagerModal to load diffs for selected staged/unstaged file rows
- Improve file-row UX with active/focus styling, keyboard activation, and guarded async diff loading/error states
- Expand dashboard route and modal tests to cover per-file diff behavior, query validation, and stage/unstage interaction isolation
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