Add ETA and rate to header sync progress

Summary

The header sync phase (post-Mithril) shows current/target slot but no ETA or sync rate, making it hard to estimate completion time.

Current Behavior

"headerSyncProgress": {
  "currentSlot": 3322558,
  "targetSlot": 103093009
}

Proposed

Match the extraction progress format:

"headerSyncProgress": {
  "currentSlot": 3322558,
  "targetSlot": 103093009,
  "percent": 3.2,
  "rate": 15000.5,
  "eta": 6652.3
}

Where:

• percent: progress percentage
• rate: slots/second
• eta: estimated seconds remaining

Implementation

Use the same sliding window approach as ExtractionProgress in Metrics/Types.hs.