feat: expandable inline diff hunk viewer with restore action

[batonogov]

Summary

When user clicks a diff marker in the gutter, expand an inline hunk viewer showing old vs new lines with a Restore button and navigation arrows between hunks.

This is step 2 of #687 — depends on #688 (clean gutter markers).

Motivation

Users need a way to review and restore individual changes directly in the editor, like Zed's inline diff.

Implementation ideas

• Click on gutter diff marker → insert temporary view below the hunk
• Show deleted lines (red background) above current lines (green background)
• Right-aligned toolbar: Restore button (reverts hunk to original), ↓↑ arrows for navigation
• Stage button can be added later when git staging support is implemented
• Use parsed hunks from GitStatusProvider (git diff output)
• Dismiss hunk viewer on click outside or Escape