DiffViewer component for inline file diffs

[ryuhmanov-m]

Parent issue: #37

Context

When the agent modifies files, changes are shown in the timeline but there's no visual diff. Users want to see what changed.

Scope

1. DiffViewer component

Create src/components/ui/DiffViewer.tsx:

• Accepts unified diff string as input
• Renders with syntax highlighting (green for additions, red for deletions)
• Supports collapsed/expanded state
• Lightweight — no heavy external dependencies (consider react-diff-viewer-continued or custom with Tailwind)

2. File change chips in messages

• The TimelineStep already tracks file_changes
• Surface changed file names as clickable chips below agent messages
• Clicking a chip opens the DiffViewer for that file's changes

References

• Timeline component: src/components/chat/
• Existing UI components: src/components/ui/