btc: warn on high all-change fees

### src/rust/bitbox02-rust/src/hww/api/bitcoin/signtx.rs
@@ -1160,16 +1160,28 @@ async fn _process(
     let fee: u64 = total_out
         .checked_sub(outputs_sum_out)
         .ok_or(Error::InvalidInput)?;
-    let fee_percentage: Option<f64> = if outputs_sum_out == 0 {
-        None
+    let (fee_percentage, fee_percentage_basis) = if outputs_sum_out == 0 {
+        // All-change and OP_RETURN-only transactions have no send amount to use as a denominator.
+        // Compare the fee to the verified input total instead, so consuming a large share of the
+        // inputs as fees still triggers the warning.
+        let fee_percentage = if inputs_sum_pass1 == 0 {
+            None
+        } else {
+            Some(100. * (fee as f64) / (inputs_sum_pass1 as f64))
+        };
+        (fee_percentage, transaction::FeePercentageBasis::TotalInputs)
     } else {
-        Some(100. * (fee as f64) / (outputs_sum_out as f64))
+        (
+            Some(100. * (fee as f64) / (outputs_sum_out as f64)),
+            transaction::FeePercentageBasis::SendAmount,
+        )
     };
-    transaction::verify_total_fee_maybe_warn(
+    transaction::verify_total_fee_maybe_warn_with_basis(
         hal,
         &format_amount(coin_params, format_unit, total_out)?,
         &format_amount(coin_params, format_unit, fee)?,
         fee_percentage,
+        fee_percentage_basis,
     )
     .await?;
     hal.ui().status("Transaction\nconfirmed", true).await;
@@ -2584,6 +2596,73 @@ mod tests {
         );
     }
 
+    #[async_test::test]
+    async fn test_high_fee_warning_total_inputs() {
+        for with_op_return in [false, true] {
+            let transaction =
+                alloc::rc::Rc::new(core::cell::RefCell::new(Transaction::new(pb::BtcCoin::Btc)));
+            {
+                let mut tx = transaction.borrow_mut();
+                tx.outputs.retain(|output| output.ours);
+                if with_op_return {
+                    tx.outputs.push(pb::BtcSignOutputRequest {
+                        r#type: pb::BtcOutputType::OpReturn as _,
+                        payload: b"metadata".to_vec(),
+                        ..Default::default()
+                    });
+                }
+            }
+            mock_host_responder(transaction.clone());
+            mock_unlocked();
+            let init_request = transaction.borrow().init_request();
+
+            let mut mock_hal = TestingHal::new();
+            assert!(process(&mut mock_hal, &init_request).await.is_ok());
+
+            assert!(mock_hal.ui.screens.contains(&Screen::TotalFee {
+                total: "13.39999900 BTC".into(),
+                fee: "13.39999900 BTC".into(),
+                longtouch: false,
+            }));
+            assert!(
+                mock_hal
+                    .ui
+                    .contains_confirm("High fee", "The fee is 66.0%\nof all inputs.\nProceed?")
+            );
+            assert_eq!(
+                mock_hal.ui.contains_confirm("OP_RETURN", "metadata"),
+                with_op_return
+            );
+        }
+    }
+
+    #[async_test::test]
+    async fn test_no_high_fee_warning_total_inputs_below_threshold() {
+        let transaction =
+            alloc::rc::Rc::new(core::cell::RefCell::new(Transaction::new(pb::BtcCoin::Btc)));
+        {
+            let mut tx = transaction.borrow_mut();
+            tx.outputs.retain(|output| output.ours);
+            tx.outputs.truncate(1);
+            tx.outputs[0].value = 2_000_000_000;
+        }
+        mock_host_responder(transaction.clone());
+        mock_unlocked();
+        let init_request = transaction.borrow().init_request();
+
+        let mut mock_hal = TestingHal::new();
+        assert!(process(&mut mock_hal, &init_request).await.is_ok());
+
+        assert!(mock_hal.ui.screens.contains(&Screen::TotalFee {
+            total: "0.30000000 BTC".into(),
+            fee: "0.30000000 BTC".into(),
+            longtouch: true,
+        }));
+        assert!(!mock_hal.ui.screens.iter().any(|screen| {
+            matches!(screen, Screen::Confirm { title, .. } if title == "High fee")
+        }));
+    }
+
     // Test a P2TR output. It is not part of the default test transaction because Taproot is not
     // active on Litecoin yet.
     #[async_test::test]

### src/rust/bitbox02-rust/src/workflow/transaction.rs
@@ -11,25 +11,62 @@ fn format_percentage(p: f64) -> String {
     util::decimal::format_no_trim(int, 1)
 }
 
+/// The denominator used to calculate and describe a transaction's fee percentage.
+pub enum FeePercentageBasis {
+    /// The amount sent to non-change/send outputs.
+    SendAmount,
+    /// The sum of the transaction's verified input values.
+    TotalInputs,
+}
+
+impl FeePercentageBasis {
+    fn warning_message(&self, fee_percentage: &str) -> String {
+        match self {
+            FeePercentageBasis::SendAmount => {
+                format!("The fee is {}%\nthe send amount.\nProceed?", fee_percentage)
+            }
+            FeePercentageBasis::TotalInputs => {
+                format!("The fee is {}%\nof all inputs.\nProceed?", fee_percentage)
+            }
+        }
+    }
+}
+
 pub async fn verify_total_fee_maybe_warn(
     hal: &mut impl crate::hal::Hal,
     total: &str,
     fee: &str,
     fee_percentage: Option<f64>,
+) -> Result<(), UserAbort> {
+    verify_total_fee_maybe_warn_with_basis(
+        hal,
+        total,
+        fee,
+        fee_percentage,
+        FeePercentageBasis::SendAmount,
+    )
+    .await
+}
+
+pub async fn verify_total_fee_maybe_warn_with_basis(
+    hal: &mut impl crate::hal::Hal,
+    total: &str,
+    fee: &str,
+    fee_percentage: Option<f64>,
+    fee_percentage_basis: FeePercentageBasis,
 ) -> Result<(), UserAbort> {
     const FEE_WARNING_THRESHOLD: f64 = 10.;
     let fee_percentage = fee_percentage.filter(|&f| f >= FEE_WARNING_THRESHOLD);
     let longtouch = fee_percentage.is_none();
     hal.ui().verify_total_fee(total, fee, longtouch).await?;
 
     if let Some(fee_percentage) = fee_percentage {
+        let warning_message =
+            fee_percentage_basis.warning_message(&format_percentage(fee_percentage));
         hal.ui()
             .confirm(&ConfirmParams {
                 title: "High fee",
-                body: &format!(
-                    "The fee is {}%\nthe send amount.\nProceed?",
-                    format_percentage(fee_percentage)
-                ),
+                body: &warning_message,
                 longtouch: true,
                 ..Default::default()
             })