feat: Add Bitcoin-Core style -walletnotify support

[craftsoldier]

This PR introduces native -walletnotify support to zecd, matching the well-known behavior of Bitcoin Core.

Many node operators, exchanges, and payment processors rely on -walletnotify to trigger external webhooks or scripts (e.g., syncing transactions to a database like Supabase) without needing to set up heavy polling architectures.

Changes

• Added walletnotify to AppConfig, ConfigFile, and Cli (configurable via zecd.toml or --walletnotify="script %s").
• Propagated the config down to WalletActor via ActorConfig.
• Added a trigger_walletnotify helper function to spawn a detached background thread (std::thread::spawn) executing the command via std::process::Command::new("sh"), safely replacing %s with the transaction ID.
• Hooked the trigger into the three primary discovery paths:
  • mempool_sync_step (for 0-conf incoming unmined transactions).
  • Enhancement block (for newly mined 1-conf shielded transactions).
  • TransactionsInvolvingAddress block (for newly mined 1-conf transparent transactions).

Implementation Details

The execution is intentionally offloaded to a detached thread so it does not block zecd's main asynchronous event loop or freeze background sync.

Happy to make any adjustments to match the project's preferred styling or architecture!

[emersonian]

This is a great idea! Reviewing and will get this merged, really appreciate the contribution.

[craftsoldier]

Cheers from the zcashme team! We use zecd a lot internally and hope you get funded at the Q3 retrospective grant. This is a fully functioning wallet you built solo without the ZODL team's help — cheers!