feat(static): improve nextjs static example

- upgrade all dependencies
- upgrade to Tailwind CSS v4
- remove ugly default background
- remove unnecessary default content
- clean up files and remove those that are no longer needed

nextjs/spa/postcss.config.mjs

@@ -0,0 +1,6 @@
+const config = {
+    plugins: {
+        "@tailwindcss/postcss": {},
+    },
+};
+export default config;