Loads user preferences from the designated file.

Returns:

• {:ok, preferences_map} if successful.
• {:error, :file_not_found} if the file doesn't exist.
• {:error, reason} for other file or decoding errors.

Returns the full path to the user preferences file. Uses an application-specific directory.

Saves the given preferences map to the designated file.

Serializes the map using :erlang.term_to_binary.

Returns:

• :ok on success.
• {:error, reason} on failure.