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 */

frontend/messages/de.json

@@ -707,6 +707,7 @@
 	"error_invite_revoked": "Dieser Einladungslink wurde deaktiviert.",
 	"error_invite_expired": "Dieser Einladungslink ist abgelaufen.",
 	"error_group_has_active_invites": "Diese Gruppe kann nicht gelöscht werden, da sie in einer aktiven Einladung verwendet wird.",
+	"error_group_not_found": "Die angegebene Gruppe existiert nicht.",
 	"register_heading": "Konto erstellen",
 	"register_subtext": "Du wurdest eingeladen, dem Familienarchiv beizutreten.",
 	"register_label_first_name": "Vorname",

frontend/messages/en.json

@@ -707,6 +707,7 @@
 	"error_invite_revoked": "This invite link has been deactivated.",
 	"error_invite_expired": "This invite link has expired.",
 	"error_group_has_active_invites": "This group cannot be deleted because it is referenced by one or more active invite links.",
+	"error_group_not_found": "The specified group does not exist.",
 	"register_heading": "Create account",
 	"register_subtext": "You've been invited to join Familienarchiv.",
 	"register_label_first_name": "First name",

frontend/messages/es.json

@@ -707,6 +707,7 @@
 	"error_invite_revoked": "Este enlace de invitación ha sido desactivado.",
 	"error_invite_expired": "Este enlace de invitación ha expirado.",
 	"error_group_has_active_invites": "Este grupo no puede eliminarse porque está referenciado por uno o más enlaces de invitación activos.",
+	"error_group_not_found": "El grupo especificado no existe.",
 	"register_heading": "Crear cuenta",
 	"register_subtext": "Has sido invitado a unirte al Familienarchiv.",
 	"register_label_first_name": "Nombre",

frontend/src/lib/shared/errors.ts

@@ -23,6 +23,7 @@ export type ErrorCode =
 	| 'INVITE_REVOKED'
 	| 'INVITE_EXPIRED'
 	| 'GROUP_HAS_ACTIVE_INVITES'
+	| 'GROUP_NOT_FOUND'
 	| 'ANNOTATION_NOT_FOUND'
 	| 'ANNOTATION_UPDATE_FAILED'
 	| 'TRANSCRIPTION_BLOCK_NOT_FOUND'
@@ -111,6 +112,8 @@ export function getErrorMessage(code: ErrorCode | string | undefined): string {
 			return m.error_invite_expired();
 		case 'GROUP_HAS_ACTIVE_INVITES':
 			return m.error_group_has_active_invites();
+		case 'GROUP_NOT_FOUND':
+			return m.error_group_not_found();
 		case 'ANNOTATION_NOT_FOUND':
 			return m.error_annotation_not_found();
 		case 'ANNOTATION_UPDATE_FAILED':