refactor: upgrade to TailwindCSS v4

[adrianthedev]

Description

Fixes #3631

This PR is upgrading Tailwind from v3 to v4.

Rollout

We'll release 3.17.1 with the Tailwind (TW) version 3 and a new 3.17.1.tw4 with TW version 4 in parallel.
We ask the community to try the tw.4 version and report back any inconsistencies and bugs they find.
Then, in a future version (probably 3.18) we'll merge this PR to main and switch all apps to TailwindCSS v4.

Help us test it 🙌

Please use the tw4 version and check for any inconsistencies. Ideally, you'll not see any differences from the past versions. This is intended. Basically we want to make sure the transition from TW 3 to 4 is seamless without any changes. Then we'll be able to improve it iteratively.
Thank you for your help!

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

Screenshots & recording

Manual review steps

1. Check the app for visual inconsistencies that weren't previously there

Manual reviewer: please leave a comment with output from the test if that's the case.

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