Challenge: Reshaping Data

You are given a data frame in wide format, where each row represents a subject and each column (apart from the first) represents a measurement variable.

• Convert the input data frame from wide format to long format so that each measurement for each subject appears as a separate row. The resulting long format data frame should have columns for the subject, the variable name, and the value.
• After reshaping, calculate the mean value for each variable across all subjects. Store these results in a summary data frame with one row per variable and columns for the variable name and its mean value.
• Return a list with two elements: the reshaped long format data frame (named long_df) and the summary data frame (named summary_df).
• Ensure the column names in the long format data frame are: Subject, variable, and value. The summary data frame should have columns: variable and value (the mean).
• Use only base R functions for data manipulation.