- make each month a card-sized accordion (single-open, first month
expanded by default) instead of a sticky header over flat rows
- turn the floating pill into a month-year selector: opens a menu of
all months and expands + scrolls to the chosen month
- remove tags from expense rows again until a placement is settled
- remove unused @mui/x-data-grid@7, which pulled @mui/system@7 over
material's v5 and caused the createTheme runtime crash
