feat(person): add BIRTH_AFTER_DEATH and INVALID_DATE_PRECISION error codes

Backend enum, frontend ErrorCode mirror, getErrorMessage cases, and
error message i18n keys (de/en/es) incl. the mixed-precision workaround
hint in error_birth_after_death.

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

backend/src/main/java/org/raddatz/familienarchiv/exception/ErrorCode.java

@@ -15,6 +15,10 @@ public enum ErrorCode {
     ALIAS_NOT_FOUND,
     /** The submitted personType value is not allowed (e.g. SKIP is import-only). 400 */
     INVALID_PERSON_TYPE,
+    /** A person's birth date is after their death date. 400 */
+    BIRTH_AFTER_DEATH,
+    /** A life date and its precision are incoherent: date present with UNKNOWN precision, or precision set without a date. 400 */
+    INVALID_DATE_PRECISION,
     // --- Documents ---
     /** A document with the given ID does not exist. 404 */
     DOCUMENT_NOT_FOUND,