Fix Issue (#1228): Eliminate offchainRegistry

[dimension-drifter]

Problem:
Currently, the wallet maintains a separate offChainRegistry to track off-chain transactions and boxes. This registry is updated when:

• New transactions enter the mempool (ScanOffChain message)
• Blocks are applied (updateOnBlock)

However, the wallet is not notified when transactions are removed from the mempool due to:

• Being invalidated
• Being replaced by higher-fee double-spends
• Expiring from the mempool

This causes the offChainRegistry to become stale, showing boxes that are no longer available in the mempool.

Solution:
Eliminate offChainRegistry entirely and query the mempool directly when off-chain boxes are needed. The wallet already receives ChangedMempool events and maintains mempoolReaderOpt, so we can:

• Extract off-chain boxes from mempoolReaderOpt on-demand
• Remove the offChainRegistry field from ErgoWalletState
• Remove OffChainRegistry class
• Simplify WalletScanLogic.scanBlockTransactions to only update WalletRegistry
• Remove ScanOffChain message handling (no longer needed - mempool updates are sufficient)

Changes:

• Modified ErgoWalletState to remove offChainRegistry field
• Added helper methods to extract off-chain boxes directly from mempool
• Updated ErgoWalletService methods to use mempool directly
• Removed OffChainRegistry class
• Updated WalletScanLogic to only return WalletRegistry updates
• Removed ScanOffChain handling from ErgoWalletActor
• Updated all tests