Add Dashboard UI for storage and deduplication statistics
New components:
- Dashboard.tsx - main dashboard page with stats overview
- Dashboard.css - responsive styling with dark theme
Features:
- Storage overview cards (total used, saved, ratio, percentage)
- Artifact statistics cards
- Deduplication effectiveness visualization with progress bars
- Circular progress indicator for deduplication rate
- Top referenced artifacts table
Updated files:
- api.ts - added getStats, getDeduplicationStats, getTimelineStats, getCrossProjectStats
- types.ts - added Stats, DeduplicationStats, TimelineStats interfaces
- App.tsx - added /dashboard route
- Layout.tsx - added Dashboard navigation link
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