Is it from the Sunnah to put a grain of salt or some salt on food, whether sweets, cooked dishes, or any other kind?

The hadith "Salt is the master of your condiments" is weak. Even if it were authentic, it would not be in the sense of a Sunnah, but rather a praise of salt and its benefits, similar to the hadith "Vinegar is an excellent condiment." The intent behind these is to praise salt and vinegar, and similar readily available and easy-to-obtain items, encouraging contentment with them rather than pursuing the desires of the self. The use of salt is a matter of nature and custom and falls under permissible things (mubahat); it has no connection to Sunnah or innovation (bid'ah). The Prophet, peace and blessings be upon him, used to leave some permissible foods and like others. His actions, peace and blessings be upon him, that are ingrained in human nature, such as eating and drinking, are not legislative or a Sunnah to be emulated.