fix(user): replace Math.abs(hashCode()) with Math.floorMod in computeColor

Math.abs(Integer.MIN_VALUE) overflows back to Integer.MIN_VALUE (negative),
making the old pattern unsafe for any palette size that doesn't evenly divide
MIN_VALUE. Math.floorMod always returns a non-negative residue in [0, n-1],
eliminating the overflow edge case entirely.

Fixes SpotBugs RV_ABSOLUTE_VALUE_OF_HASHCODE (priority 1, CORRECTNESS).
Closes #471

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/user/AppUser.java

@@ -88,7 +88,8 @@ public class AppUser {
     };
 
     public static String computeColor(UUID id) {
-        return PALETTE[Math.abs(id.hashCode()) % PALETTE.length];
+        // Math.floorMod avoids the Integer.MIN_VALUE overflow trap in Math.abs(hashCode())
+        return PALETTE[Math.floorMod(id.hashCode(), PALETTE.length)];
     }
 
     @PrePersist

backend/src/test/java/org/raddatz/familienarchiv/user/AppUserTest.java

@@ -35,4 +35,15 @@ class AppUserTest {
             .count();
         assertThat(distinct).isGreaterThan(1);
     }
+
+    @Test
+    void computeColor_returnsValidPaletteColorForIntegerMinValueHash() {
+        // UUID "80000000-0000-0000-0000-000000000000" has hashCode() == Integer.MIN_VALUE.
+        // Math.abs(Integer.MIN_VALUE) overflows back to Integer.MIN_VALUE (negative), making
+        // Math.abs(hashCode()) % n unsafe for palette sizes that don't evenly divide MIN_VALUE.
+        // Math.floorMod eliminates this edge case entirely.
+        UUID minHashId = UUID.fromString("80000000-0000-0000-0000-000000000000");
+        assertThat(minHashId.hashCode()).isEqualTo(Integer.MIN_VALUE);
+        assertThat(EXPECTED_PALETTE).contains(AppUser.computeColor(minHashId));
+    }
 }