Adopt diff2html for Diff Viewer with Dark Theme Support

[Copilot]

Replaced the custom diff viewer implementation with the robust diff2html library to provide a more professional and feature-rich diff experience, with proper dark theme integration and layout fixes.

Changes Made

• Replaced custom diff rendering with diff2html library integration
• Added unified diff generation function to convert existing FileDiff data to diff2html compatible format
• Maintained backward compatibility - existing diffData structures continue to work unchanged
• Removed unused components - cleaned up FileDiffView component and associated files
• Enhanced visual presentation with syntax highlighting and professional styling
• Fixed styling issues - Resolved line number positioning and dark theme compatibility problems

Implementation Details

The solution maintains the existing API by:

1. Keeping the current generateFilesDiff() and FileDiff data structures intact
2. Adding a new generateUnifiedDiff() function that converts our data to unified diff format
3. Using diff2html to render the unified diff as HTML in the React component
4. Preserving the summary statistics (file count, additions, deletions) in the UI

Styling Fixes

The initial implementation had layout issues where line numbers appeared disconnected from the diff content. This was resolved by:

• CSS override system - Added comprehensive CSS overrides to fix position: absolute conflicts in flexbox layouts
• Dark theme variables - Manually configured diff2html's CSS custom properties to match the application's dark theme
• Layout corrections - Forced proper table layout for line number alignment instead of relying on absolute positioning
• Color scheme consistency - Set explicit colorScheme: 'dark' for reliable dark theme rendering

Visual Result

The new implementation provides:

• Professional diff formatting with clear visual indicators
• Proper line number alignment that stays connected to the diff content
• Full dark theme support that integrates seamlessly with the application's design
• Syntax highlighting for better code readability
• Improved user experience with modern styling
• Robust functionality through the mature diff2html library

All existing tests pass, confirming no regressions were introduced.

Fixes #984.

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[curran]

Does not work well. The styling is completely messed up.

The line numbers appear below all the content, disconnected. Also, we need to theme it for dark mode.

[Copilot]

Does not work well. The styling is completely messed up.

...

Fixed the styling issues! The line number positioning problems were caused by CSS conflicts between diff2html's position: absolute layout and the application's flexbox structure.

The solution:

• Added CSS overrides to force proper table layout for line numbers
• Configured explicit dark theme variables to match the application's color scheme
• Set colorScheme: 'dark' explicitly instead of relying on auto-detection

Commit: a20d1ad

The diff viewer should now display correctly with proper line number positioning and full dark theme support.