Git history graph: navigate to specific commit diff on click

[parsakhaz]

Context

PR #16 added a git history graph to the DetailPanel sidebar. Clicking a commit row switches to the diff panel tab, but the selected commit hash is discarded — the diff panel still shows whatever diff was previously active.

Enhancement

When a user clicks a commit in the history graph, the diff panel should load and display that specific commit's diff rather than the current default view.

What's needed

• Extend the diff panel to accept a target commit hash
• Wire handleSelectCommit in DetailPanel.tsx to pass the hash through to the diff panel
• Update the diff panel's data fetching to support single-commit diffs

References

• frontend/src/components/DetailPanel.tsx — handleSelectCommit (line ~36-42)
• frontend/src/components/GitHistoryGraph.tsx — onSelectCommit callback