Add Display for Euros

Renders as "123.45 €" — dot separator and a plain space, i.e. American
style. French formatting (narrow no-break space, comma separator) will need
a separate path since Display takes no parameters; deferred until there is
a page to render.

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

crates/immo-core/src/euros.rs

@@ -1,3 +1,5 @@
+use std::fmt;
+
 use rust_decimal::Decimal;
 
 #[derive(PartialOrd, PartialEq, Ord, Eq, Clone, Copy, Debug)]
@@ -19,6 +21,12 @@ impl Euros {
     }
 }
 
+impl fmt::Display for Euros {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{} €", self.as_decimal())
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -77,6 +85,25 @@ mod tests {
     #[test]
     fn from_0_cents_to_decimal_stays_zero() {
         let euros = Euros::from_cents(0);
+        assert_eq!(euros.as_decimal().scale(), 2);
         assert_eq!(euros.as_decimal(), Decimal::ZERO);
     }
+
+    #[test]
+    fn standard_euros_prints_correctly() {
+        let euros = Euros::from_cents(123_45);
+        assert_eq!(format!("{}", euros), "123.45 €");
+    }
+
+    #[test]
+    fn euros_without_cents_prints_with_trailing_zeros() {
+        let euros = Euros::from_cents(123_00);
+        assert_eq!(format!("{}", euros), "123.00 €");
+    }
+
+    #[test]
+    fn euros_with_negative_amount_print_with_minus() {
+        let euros = Euros::from_cents(-123_00);
+        assert_eq!(format!("{}", euros), "-123.00 €");
+    }
 }