refactor(web-extension): reduce cognitive complexity in SigningCoordinator#signRequest
- Extract #findAccount method for account lookup logic - Extract #createInMemorySignRequest for in-memory wallet sign requests - Extract #createHardwareSignRequest for hardware wallet sign requests - Extract #createHardwareKeyAgent for key agent creation logic - Replace complex nested ternary with simple conditional - Add proper TypeScript types for Bip32WalletAccount - Remove eslint-disable comment for cognitive complexity - Maintain exact same functionality while improving readability