feat: migrate to Tailwind CSS v4

Follows docs/tailwind-v4-migration.md: installs @tailwindcss/postcss
and points Angular's PostCSS pipeline at it via .postcssrc.json, swaps
the @tailwind base/components/utilities directives in styles.css for
@import "tailwindcss", and bridges the existing tailwind.config.js
(darkMode: "class", content globs) via @config so nothing else needs
to change. Drops autoprefixer, which v4 now handles internally.

Verified with a production build, the full test suite, and a visual
check of the dev server in both light and dark mode.

Author: samuelsonbrito

.postcssrc.json

@@ -0,0 +1,5 @@
+{
+  "plugins": {
+    "@tailwindcss/postcss": {}
+  }
+}