Add withdrawal address validator dashboard

[JordyDutch]

Summary

• Add a withdrawal-address dashboard for validator performance
• Support lookup by withdrawal address or withdrawal credentials
• Show validator counts, active/online/offline status, pending queue
  breakdown, balances, and validator rows
• Add withdrawal-address filtering support for validator list and activity/
  offline views
• Add pending validator handling for Electra deposit queue data
• Add a focused withdrawal credentials index for filtered validator lookups

Implementation Notes

• Keeps the implementation additive and aligned with existing Dora handler/
  template patterns
• Reuses existing validator, deposit queue, liveness, and frontend cache
  services
• Avoids broad refactors and keeps the dashboard isolated behind a new
  route

Testing

• go test ./handlers ./services ./utils

Screenshots

Dashboard

Filter option validator overview

[blazejkrzak]

Finally some great feature!