Code-review follow-ups on Task 5:
1. Chevron rotation — Bootstrap 5 collapse sets aria-expanded="false" on
the toggle when collapsed; two CSS rules off that attribute rotate
the chevron -90deg with a 150ms transition. No JS needed.
2. test_group_by_type now asserts groups[0]['label'] == 'Bonus', locking
in the descending-|net_sum| ordering promise (|800| > |100|).
60/60 tests still pass.
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