Upgrade to TailwindCSS v4

[emmadesilva]

See https://tailwindcss.com/docs/upgrade-guide#using-the-upgrade-tool

All existing projects should run:

$ npx @tailwindcss/upgrade

We now use https://tailwindcss.com/blog/tailwindcss-v4#first-party-vite-plugin. As a side effect, we needed to switch to the ESM extension for the Vite configuration https://vite.dev/guide/troubleshooting.html#this-package-is-esm-only

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (4b63f4d) to head (3b90e57).
Report is 9 commits behind head on 2.x-dev.

Additional details and impacted files

@@             Coverage Diff             @@
##             2.x-dev     #2146   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity      1976      1976           
===========================================
  Files            199       199           
  Lines           5282      5282           
===========================================
  Hits            5282      5282           

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