🤖 fix: show old/new line numbers in diffs

[ammar-agent]

Aligns diff viewer with GitHub's visual/interaction model:

• Dual line-number columns (old/new) to clarify pre- vs post-change lines
• Review note formatting includes both ranges (e.g. -12-14 +15-17)
• Restored +/− indicators with hover-to-replace review button (MessageSquare)
• Deduplicated rendering via shared DiffLineGutter and DiffIndicator components
• Polished review input styled to sit within the diff seamlessly, background matches surrounding line type (add/remove/context)

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