feat: use Amount on calculate_fee, fee_absolute, fee_amount and others

- update to use `bitcoin::Amount` on `CreateTxError::FeeTooLow` variant.
- update to use `bitcoin::Amount` on `Wallet::calculate_fee()`.
- update to use `bitcoin::Amount` on `FeePolicy::fee_absolute()`.
- update to use `bitcoin::SignedAmount` on
  `CalculateFeeError::NegativeFee` variant.
- update to use `bitcoin::Amount` on `TxGraph::calculate_fee()`.
- update to use `bitcoin::Amount` on `PsbUtils::fee_amount()`

crates/electrum/tests/test_electrum.rs

@@ -101,7 +101,8 @@ fn scan_detects_confirmed_tx() -> anyhow::Result<()> {
             .fee
             .expect("Fee must exist")
             .abs()
-            .to_sat() as u64;
+            .to_unsigned()
+            .expect("valid `SignedAmount`");
 
         // Check that the calculated fee matches the fee from the transaction data.
         assert_eq!(fee, tx_fee);