feat: redesign diff view with horizontal layout and new tab options

[ThomasK33]

Improved Diff View Layout and Configuration

This PR refactors the diff view functionality to provide more flexible layout options and better tab management:

• Added a new layout option to replace the previous vertical_split boolean, supporting both "vertical" and "horizontal" layouts
• Replaced open_in_current_tab with open_in_new_tab for more intuitive configuration
• Removed redundant options auto_close_on_accept and show_diff_stats that were no longer used
• Added proper type annotations for configuration options
• Implemented improved window management when opening diffs in new tabs
• Added terminal window preservation when opening diffs in new tabs
• Improved window option handling to maintain consistent appearance across different layouts
• Enhanced cleanup logic to properly handle both tab-based and window-based diff views
• Updated tests to reflect the new configuration options

These changes make the diff view more customizable while simplifying the configuration interface. Users can now choose between vertical or horizontal layouts and easily open diffs in new tabs while maintaining access to the terminal.

[ThomasK33]

• feat: redesign diff view with horizontal layout and new tab options #111 👈 (View in Graphite)
• feat: add Claude Haiku model and update type annotations #110
• main

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