refactor: deduplicate PaneLeafView and ContentView shared logic

## Summary

Follow-up from PR #645. ~300 lines duplicated between PaneLeafView and ContentView (git diff/blame, close confirmation, gutter accept/revert).

## Motivation

Code duplication leads to bugs when one copy is updated but not the other.

## Implementation ideas

- Extract shared logic into a helper/protocol
- Or use a shared ViewModel