Use borrowed &str keys for existing_tx_map in process_wallet_transactions

[schjonhaug]

Context

In sync.rs, the existing_tx_map HashMap is built in three places with inconsistent key types:

Location	Function	Key type
~line 414	process_wallet_transactions	HashMap<String, &_> (clones each txid)
~line 1545	check_address_watch_for_confirmations	HashMap<&str, &_> (borrows, zero-copy)
~line 1832	check_grouped_address_watch_for_confirmations	HashMap<&str, &_> (borrows, zero-copy)

The &str-keyed maps avoid heap allocation per entry. The main process_wallet_transactions function should use the same &str borrowing pattern for consistency and to avoid unnecessary String clones.

Proposed fix

Change line ~414 from:

let existing_tx_map: HashMap<String, &TransactionWithWallet> = existing_transactions
    .iter()
    .map(|tx| (tx.txid.clone(), tx))
    .collect();

To:

let existing_tx_map: HashMap<&str, &TransactionWithWallet> = existing_transactions
    .iter()
    .map(|tx| (tx.txid.as_str(), tx))
    .collect();

This requires updating lookup call sites to use .get(txid.as_str()) instead of .get(txid) where the key is a String.

Identified during PR #245 code review.