fix: date and datetime regression (#282)

* use custom types.Date implementation

* fix user registration bug

* remove sanity check

* fix datetime bug

backend/app/api/static/docs/docs.go

@@ -1983,6 +1983,7 @@ const docTemplate = `{
                     "type": "string"
                 },
                 "warrantyExpires": {
+                    "description": "Sold",
                     "type": "string"
                 }
             }