feat: improve registration validation

src/main/java/com/openisle/exception/FieldException.java

@@ -0,0 +1,19 @@
+package com.openisle.exception;
+
+import lombok.Getter;
+
+/**
+ * Exception carrying a target field name. Useful for reporting
+ * validation errors to clients so they can display feedback near
+ * the appropriate input element.
+ */
+@Getter
+public class FieldException extends RuntimeException {
+    private final String field;
+
+    public FieldException(String field, String message) {
+        super(message);
+        this.field = field;
+    }
+}
+