feat(errors): add GROUP_NOT_FOUND error code + i18n keys

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

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

@@ -30,6 +30,8 @@ public enum ErrorCode {
     // --- Users ---
     /** A user with the given ID or username does not exist. 404 */
     USER_NOT_FOUND,
+    /** A group with the given ID does not exist. 404 */
+    GROUP_NOT_FOUND,
     /** The supplied email address is already used by another account. 409 */
     EMAIL_ALREADY_IN_USE,
     /** The supplied current password does not match the stored hash. 400 */