feat: inline diff hunk viewer with stage/restore actions

[batonogov]

Summary

Replace the current broken inline diff accept/revert buttons (which overlap line numbers in the gutter) with a proper inline diff hunk viewer, similar to Zed editor.

Current behavior

Modified lines show garbled accept/revert buttons overlapping line numbers in the gutter (e.g., line 37 displays as "87℃").

Expected behavior

Collapsed state:

• Yellow marker strip in the gutter next to modified lines — line numbers stay clean and readable

Expanded state (click on marker):

• Inline hunk expands showing old (deleted) line in red and new line in green
• Right-aligned action buttons: Stage, Restore
• Navigation arrows (↓ ↑) to jump between changes

Motivation

Current inline diff UI is unusable — buttons render on top of line numbers. A proper hunk viewer (like Zed) gives users a clear way to review, stage, and restore individual changes.

Implementation ideas

• Gutter markers: keep yellow/green/red strips, remove current accept/revert buttons
• On click: insert temporary rows below the changed line showing the diff hunk (old vs new)
• Action buttons (Stage, Restore) rendered in a toolbar row inside the expanded hunk
• Navigation arrows to cycle through all hunks in the file
• Use git diff parsed hunks from GitStatusProvider