UX: Add PRD version history panel for tracking requirement evolution

[frankbria]

Problem

The PRD page calls prdApi.createVersion on save, storing change_summary, but the UI only shows the current PRD content. Users have no way to view version history, compare versions, or revert to a previous state.

Impact

Teams cannot track how requirements evolved, making it impossible to understand why tasks were added/removed or audit requirement changes.

Recommendation

1. Add 'Version History' section or side panel to PRDView showing a list of saved versions with timestamps and change_summary
2. Make versions clickable to preview (read-only view)
3. Add 'Compare with current' option showing diff
4. Add 'Restore this version' button with confirmation

Acceptance Criteria

• Version list accessible from PRD page (drawer, panel, or tab)
• Each version shows timestamp and change summary
• Clicking a version opens read-only preview
• Restore action creates new version (non-destructive)
• Current version highlighted in list

Affected Components

web-ui/src/app/prd/page.tsx, web-ui/src/components/prd/PRDView.tsx

[coderabbitai]

🔗 Related PRs

#194 - Fix PRD generation progress tracking UI [merged]
#196 - fix(discovery): Synchronize PRD buttons and fix state persistence on tab revisit [merged]

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[traycerai]

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Reference ID: 1558bc4a-8469-469b-bd5f-9c7a14a8341c

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