feat: CSV export for block producers by time period

[dannywillems]

Summary

Add the ability to export block producer data as a CSV file for a selected time period.

Use Case

Users need to analyze block production patterns for:

• Identifying active validators
• Comparing producer performance
• Research and analytics
• Delegation decisions

Requirements

Data to Include

• Public key
• Number of blocks produced
• Percentage of total blocks in period
• First block timestamp
• Last block timestamp
• Total coinbase earned

Filtering Options

• Date range picker (from/to)
• Preset ranges: Last 24h, 7 days, 30 days, epoch
• Minimum blocks threshold (optional)

UI Location

• Staking page (/staking)
• Add "Export Block Producers CSV" button near the producers table

CSV Format Example

public_key,blocks_produced,percentage,first_block_time,last_block_time,coinbase_earned
B62qp...,45,2.5%,2025-01-01T00:00:00Z,2025-01-15T23:59:00Z,32400.0
B62qr...,38,2.1%,2025-01-02T10:00:00Z,2025-01-15T20:30:00Z,27360.0

Acceptance Criteria

• Export button visible on staking page
• Date range filter works correctly
• CSV downloads with descriptive filename
• Sorted by blocks produced (descending)
• Percentage calculated correctly for selected period