fix: preserve signed electrum unconfirmed balance

[noahjoeris]

Problem

blockchain.scripthash.get_balance returns unconfirmed as a signed amount. When confirmed coins are spent in the mempool it can be negative. At the moment we simply turn it into a positive amount. Meaning if we receive 1 BTC or if we spend 1 BTC both is shown the same way.
Example: we spend 100 confirmed sats, this results in -100 unconfirmed sats. If we turn it positive a user can think he has now 200 sats (100 sats incoming).

Solution

• Type unconfirmed as SignedAmount and deserialize with bitcoin::amount::serde::as_sat (preserve the sign).
• I also switch other sat/BTC amount fields to bitcoin’s as_sat / as_btc helpers and drop the local equivalents.

[caarloshenriq]

tACK 4c4704e

Reproduced on master: {"confirmed":0,"unconfirmed":-100} deserializes to +100 sat, and on this branch it stays -100 sat. I also A/B tested the second commit against the removed helpers across the sat and BTC paths (zero, negative, i64 bounds, sub satoshi precision, above max money, wrong JSON types) and acceptance and rejection are identical on both sides, so the refactor is behavior preserving.

One thing before merge. There is no test covering the fix: tests/synopsis.rs only prints GetBalanceResp and only ever produces a positive unconfirmed delta, so nothing guards this. Something like:

#[test]
fn get_balance_preserves_negative_unconfirmed() {
    let resp: GetBalanceResp =
        serde_json::from_str(r#"{"confirmed":0,"unconfirmed":-100}"#).unwrap();
    assert_eq!(resp.unconfirmed, SignedAmount::from_sat(-100));
}