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