- Side-by-side diff: toggle button (columns-2/align-justify icon) in toolbar switches between unified and side-by-side view - Backend: diff endpoint returns left_content + right_content for side-by-side - Frontend: SequenceMatcher-based LCS diff for two-column rendering - CSS: .backup-diff-sidebyside with left/right borders, empty cell styling - Auto-save: PUT /save?backup=false skips backup creation (auto-save=no backup) - Footer text updated to reflect backup-on-manual-save behavior