Context. Daily sales come in as separate DataFrames; combine them into a single table.
Your task. Write combine(dfs) that takes a list of DataFrames and returns them concatenated vertically with the index reset to 0..N-1.
Example. combine([df_mon, df_tue, df_wed]) → all rows stacked, re-indexed.
Notes. Empty list → empty DataFrame (pd.concat([]) raises, so handle that). All dfs share the same columns.