#387 feat(desktop): add editable Monaco diff view for uncommitted changes

Summary

• Enable editing files directly in the Monaco diff view for unstaged and staged changes
• Added saveFile tRPC mutation to write file contents to disk
• Modified side of DiffEditor is now editable with ⌘S / Ctrl+S to save
• Shows "Editable ⌘S to save" badge in the diff toolbar when editable
• When saving from staged view, automatically switches to unstaged view (since edits become new unstaged changes)

Test plan

• Open a workspace with uncommitted changes
• Select an unstaged file - verify "Editable" badge appears and you can edit the right side
• Make changes and press ⌘S - verify file saves and diff updates
• Select a staged file - verify you can also edit it
• Save changes in staged view - verify it switches to unstaged view showing new changes

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features
  • Added in-place file editing directly in the changes view.
  • Implemented save functionality to persist edited files to disk.
  • Added visual indicators for edit mode and saving status.
  • Introduced keyboard shortcut (Ctrl/Cmd+S) for quick saving.

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