feat(sidebar): File Diff section, diff viewer, and git graph fix

[iptoux]

Summary

• Adds a File Diff sidebar section between Project Files and Git Commits: lists changed files with status markers and +/- line counts, stage/unstage actions, and auto-refresh via a debounced filesystem watcher (git_status_dirty).
• Opens an inline diff viewer as a center tab (CenterTabKind::FileDiff) for the selected file (staged or unstaged).
• Refactors the sidebar panels area to a three-slot layout (Explorer / Diff / Graph) with a second resizer and persisted height.
• Git Commits now auto-refreshes on the same dirty event (shared watcher).
• Fixes git log --graph failing because -c log.graphWidth=… was passed as one argv token instead of -c + log.graphWidth=… (symptom: “Could not load commit history” while File Diff showed a clean tree).

Test plan

• Open a workspace on a Git repo; confirm File Diff lists changes and refreshes after edits (without manual refresh).
• Click a changed file → diff tab opens with correct +/- highlighting.
• Stage/unstage from row hover actions; list updates after action.
• Git Commits loads history and refreshes after a commit or checkout.
• Resize Explorer / Diff / Graph panels; reload app and confirm heights persist.
• Workspace without Git: Diff + Graph sections hidden.

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