Add timeout, status callbacks and cancellation for signers requiring human approval

[nogringo]

Some signers require human approval (NIP-46, NIP-07, etc.). Currently there's no way to:

1. Know when a request is waiting for user approval
2. Cancel a pending request
3. Handle timeout when signer is offline/unreachable
4. Distinguish between "signer unavailable" vs "invalid request"

Use case:

When signing or decrypting, the app needs to:

• Show "waiting for approval" UI
• Allow user to cancel
• Retry later if signer was unavailable
• Handle permanent failures differently from temporary ones