expense list: month accordions + month-year selector; drop unused x-data-grid

- 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

react-openapi/package.json

@@ -11,7 +11,6 @@
     "react-router-dom": "^6.28.0",
     "@mui/material": "^5.16.7",
     "@mui/icons-material": "^5.16.7",
-    "@mui/x-data-grid": "^7.22.0",
     "@emotion/react": "^11.13.3",
     "@emotion/styled": "^11.13.0",
     "axios": "^1.7.7",