feat: Global stats dashboard

[sagarsrc]

Scope

backend, frontend

What do you want?

Aggregate statistics across all sessions to show global usage patterns and totals.

Backend:

• API endpoint for global metrics (/api/stats or /api/metrics/global)
• Aggregate totals: total tokens, total cost, total sessions, total time
• Time-series data: daily/weekly/monthly trends
• Top tools used across all sessions
• Average session duration, tokens per session

Frontend:

• Global stats view/page in dashboard
• Charts: usage over time, cost trends, tool distribution
• Summary cards: lifetime totals
• Date range filters (last 7 days, 30 days, all time)
• Comparison view (this week vs last week)

Metrics to track:

• Total tokens (input/output/cache)
• Total cost
• Total sessions
• Total time spent
• Tool usage breakdown
• Model usage breakdown

Why?

Users want to see their overall AI usage patterns beyond individual sessions. Global stats help track spending, identify trends, and understand long-term productivity with Claude Code.