Add commit attestation chart to StatsCard

[cyberstorm-nisto]

Goal

StatsCard currently shows identity registrations only. Add commit/contribution attestations.

Current

• Queries Identity Schema for registration count + history
• Shows time-series chart of registrations

Needed

• Query Contribution Schema (0x7425c71616d2959f30296d8e013a8fd23320145b1dfda0718ab0a692087f8782)
• Show:
  • Total commit attestations
  • Time-series chart (separate line or combined)
  • Maybe: commits per identity (leaderboard?)

Implementation

1. Add CONTRIBUTION_SCHEMA_UID to config
2. Duplicate/extend fetchAttestationHistory for contributions
3. Add second data series to Recharts
4. Update StatsCard UI to show both metrics

Depends On

• Issue Delegated commit attestation backend #10 (backend) - need actual commit attestations to display
• For now, can show 0 or placeholder