chore(frontend): bump tailwindcss to v4 (#4665)

# Motivation

We want to bump `tailwindcss` to v4. However [the migration tool does
not support SASS
files](tailwindlabs/tailwindcss#15716).

# Workaround

We changed all the `.scss` files to a copy in `.css` and use the
migration tool, and then reverted. Same thing with the language in the
`<style>` tag of Svelte components.
Then we added a few manual tweaks anyway.

# Disclaimer

Since a part of the PR was a manual process, it may very well be that I
missed something or there is some error in migration.

# Note

Apparently `prettier-plugin-tailwindcss` was bumped too, so a few
changes happened in the sorting order of the tailwind classes too. That
is the reason for this many files changed in this PR, since we have an
auto-formatter.

Furthermore, it seems that [a few classes will require a Vite update to
work properly](vitejs/vite#18974). For example
`space-x-*` and `space-y-*`.

# Changes

- The migration tool:
  - installed `tailwindcss` v4;
  - changed `postcss.config`;
- migrated the tailwind classes that required migration (see [upgrade
guide](https://tailwindcss.com/docs/upgrade-guide));
- correctly changed `global.css` (that we reverted to `global.scss`
after).

- A few changes were missing, and we did it manually:
- The `Hero` gradient does not need a `via-` color anymore, it can
changed to `from-`.
- The util `@screen foo` is deprecated and should be replaced with
`@media (width >= theme(--breakpoint-foo))` (see [codebase of
tailwindcss
migration](https://github.com/tailwindlabs/tailwindcss/blob/121cf6bd807a35ad9814dd5444eeb3594601fff8/packages/%40tailwindcss-upgrade/src/codemods/migrate-media-screen.test.ts)).
- It is
[suggested](https://tailwindcss.com/docs/upgrade-guide#using-vite) to
install and add `@tailwindcss/vite` in `vite` (and `vitest`) configs;
- Move `@include meta.load-css(...)` inside `@layer base {...}`
(suggestion in [tailwindcss
discussion](tailwindlabs/tailwindcss#16421)).

- We create another branch to make the changes manually following the
[upgrade guide](https://tailwindcss.com/docs/upgrade-guide) and they
matched:
  - #4666

# Tests

There is not much test to help us, apart update the E2E snapshots and
manual navigation in local replica:



https://github.com/user-attachments/assets/d0ae2e36-7fff-4ff8-8829-3c93e900650a

---------

Co-authored-by: github-actions <41898282+github-actions[bot]@users.noreply.github.com>

.prettierrc

@@ -16,6 +16,7 @@
           "prettier-plugin-organize-imports",
           "prettier-plugin-tailwindcss"
         ],
+        "tailwindStylesheet": "./src/frontend/src/lib/styles/global.scss",
         "parser": "svelte",
         "htmlWhitespaceSensitivity": "strict"
       }