Longest Daily Streak

A language app records each day a learner practised, as text like "2026-03-14".

Task: write longest_streak(days) that returns the length of the longest run of back-to-back calendar days.

• The list is not sorted, and the same day may appear more than once (practising twice in a day still counts as one day).
• Runs can cross the end of a month or a year, and leap days count.
• An empty list returns 0.

28 February, 1 March and 2 March are back-to-back (2026 is not a leap year).