feat: migrate workspace to Tailwind CSS v4 (#377)

* feat: migrate workspace to Tailwind CSS v4

* refactor: move Tailwind v4 setup to CSS directives

apps/marketing/src/app/globals.css

@@ -0,0 +1,7 @@
+@import "tailwindcss";
+
+@source "../**/*.{js,jsx,ts,tsx,mdx}";
+@source "../../../../packages/ui/src/**/*.{js,jsx,ts,tsx,mdx}";
+@source "../../../../packages/email-editor/src/**/*.{js,jsx,ts,tsx,mdx}";
+
+@import "@usesend/ui/styles/globals.css";

apps/marketing/src/app/layout.tsx

@@ -1,4 +1,4 @@
-import "@usesend/ui/styles/globals.css";
+import "./globals.css";
 
 import { Inter } from "next/font/google";
 import { JetBrains_Mono } from "next/font/google";