feat(web): use tailwindcss v4

[gabrielmfern]

Summary by cubic

Upgraded the web app to Tailwind CSS v4 and migrated classnames to the new syntax. Pinned Tailwind and the PostCSS plugin to 4.1.18 and added a base CSS rule to keep v3-like border colors.

• Dependencies

  • Pinned tailwindcss to 4.1.18 in the workspace catalog and lockfile.
  • Added @tailwindcss/postcss 4.1.18 to the catalog; switched PostCSS in web and preview-server; removed autoprefixer.
  • Updated globals.css to use @import "tailwindcss" and @config; added a base border-color override.

• Refactors

  • Migrated utilities to v4: mask-[…], bg-linear-to-, dvh/dvw, z-, ring-3, outline-hidden, rounded-xs, shadow-xs, transition-discrete.
  • Adopted new size tokens/shorthands (e.g., w-50/w-90, md:max-w-160/184); replaced bracketed values where possible.
  • Updated variants/selectors and focus styles: data-placeholder:…, [&_svg]:hover:…, focus-visible/focus-within outline-hidden.

^{Written for commit 1ed28a4. Summary will update on new commits.}

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[vercel]

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Project	Deployment	Actions	Updated (UTC)
react-email	Ready	Preview, Comment	Mar 5, 2026 9:23pm
react-email-demo	Ready	Preview, Comment	Mar 5, 2026 9:23pm

[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	@​tailwindcss/​postcss@​4.1.17 ⏵ 4.1.18

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npm i https://pkg.pr.new/@react-email/preview-server@3021

commit: afda970

[cubic-dev-ai]

No issues found across 58 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
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