Blue Railroad datepicker: improve UI and block height accuracy

[magent-cryptograss]

Current State

The date-to-blockheight converter is working but has room for improvement:

1. No popup datepicker - Currently using native <input type="datetime-local"> which varies by browser. A proper datepicker widget would provide a more consistent UX.

2. Block height accuracy - Current calculation uses a fixed 12.12 second average block time:

   var blocksDiff = Math.round((refTimestamp - targetTimestamp) / 12.12);
   return refBlock - blocksDiff;

   This could be improved by:

   • Fetching actual block timestamps from an API for more precise calculation
   • Using a reference block closer to the target date
   • Accounting for block time variations over different periods

Location

extensions/BlueRailroadIntegration/resources/ext.bluerailroad.datepicker.js

Possible Improvements

• Use a JS datepicker library (flatpickr, pikaday, etc.) for consistent cross-browser UI
• Query Etherscan/Alchemy API for actual block-to-timestamp mapping
• Add visual feedback showing confidence level of the estimate
• Consider using binary search between known block timestamps for better accuracy