Fix activity not created on send: create immediately from txid, increase LDK retries

[ben-kaufman]

Problem

Sent onchain transactions sometimes never appeared in the activity list. Logs showed:
LDK timing – onchainTransactionReceived fires ~7 seconds after broadcast.
Core queue delay – handleOnchainTransactionReceived could run 5+ minutes later due to core queue congestion.
SendSuccess polling – The success sheet polls for activity for 12 attempts (5s apart) and often times out before the activity exists.

Solution

1. Immediate activity creation
   Create the sent onchain activity as soon as we have the txid from a successful send, instead of waiting for the LDK event:
   Added createSentOnchainActivityFromSendResult() to ActivityService to create an OnchainActivity from the send result.
   Call it from SendConfirmationView right after wallet.send() returns the txid.
   When LDK later emits onchainTransactionReceived, processOnchainPayment updates this activity (e.g. confirmation status).
2. More retries in processOnchainTransaction
   Increased retries from 3×300ms to 10×500ms (~4.5s total) so LDK has more time to add the payment to listPayments() before we give up.