Store the annual nominal rate as a fraction, not a percentage

The field was called annual_rate_in_percent but nothing multiplied it by 100,
so a 3,41 % loan would have been stored as 3.41 and read as 341 %. Rename it
to annual_nominal_rate, which is what it holds, and add an explicit
annual_nominal_rate_in_percent() accessor for callers that want the display
form. Drop the trailing % from the error message, which was making the same
claim.

The nominal qualifier is there because TAEG will join this struct later and
the two must never be confused.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

crates/immo-core/src/loan_offer.rs

@@ -5,7 +5,7 @@ use crate::euros::Euros;
 
 #[derive(Debug, PartialEq, Eq, Error, Clone)]
 pub enum LoanOfferError {
-    #[error("loan cannot have a negative interest rate: {0}%")]
+    #[error("loan cannot have a negative interest rate: {0}")]
     NegativeInterestRate(Decimal),
     #[error("loan duration cannot be zero")]
     ZeroLoanDuration,
@@ -16,18 +16,18 @@ pub enum LoanOfferError {
 #[derive(Debug, PartialEq, Eq)]
 pub struct LoanOffer {
     borrowed_capital: Euros,
-    annual_rate_in_percent: Decimal,
+    annual_nominal_rate: Decimal,
     loan_duration_in_months: u64,
 }
 
 impl LoanOffer {
     pub fn new(
         borrowed_capital: Euros,
-        annual_rate_in_percent: Decimal,
+        annual_nominal_rate: Decimal,
         loan_duration_in_months: u64,
     ) -> Result<Self, LoanOfferError> {
-        if annual_rate_in_percent < Decimal::ZERO {
-            Err(LoanOfferError::NegativeInterestRate(annual_rate_in_percent))
+        if annual_nominal_rate < Decimal::ZERO {
+            Err(LoanOfferError::NegativeInterestRate(annual_nominal_rate))
         } else if loan_duration_in_months == 0 {
             Err(LoanOfferError::ZeroLoanDuration)
         } else if borrowed_capital <= Euros::from_cents_as_i64(0) {
@@ -35,11 +35,15 @@ impl LoanOffer {
         } else {
             Ok(LoanOffer {
                 borrowed_capital,
-                annual_rate_in_percent,
+                annual_nominal_rate,
                 loan_duration_in_months,
             })
         }
     }
+
+    pub fn annual_nominal_rate_in_percent(&self) -> Decimal {
+        self.annual_nominal_rate * Decimal::ONE_HUNDRED
+    }
 }
 
 #[cfg(test)]
@@ -92,7 +96,7 @@ mod tests {
             ret,
             Ok(LoanOffer {
                 borrowed_capital: Euros::from_cents_as_i64(10),
-                annual_rate_in_percent: Decimal::new(45, 2),
+                annual_nominal_rate: Decimal::new(45, 2),
                 loan_duration_in_months: 15
             })
         );
@@ -105,9 +109,20 @@ mod tests {
             ret,
             Ok(LoanOffer {
                 borrowed_capital: Euros::from_cents_as_i64(10),
-                annual_rate_in_percent: Decimal::ZERO,
+                annual_nominal_rate: Decimal::ZERO,
                 loan_duration_in_months: 15
             })
         );
     }
+
+    #[test]
+    fn conversion_to_annual_nominal_rate_in_percent_works() {
+        let offer = LoanOffer::new(
+            Euros::from_cents_as_i64(150_000_00),
+            Decimal::new(4, 4),
+            300,
+        )
+        .unwrap();
+        assert_eq!(offer.annual_nominal_rate_in_percent(), Decimal::new(4, 2));
+    }
 }