feat: scaffold Next.js 16 web app with Tailwind v4

Sets up apps/web with Next.js 16, React 19, Tailwind CSS v4 (postcss plugin
config, @import syntax), Jest + Testing Library, App Router layout and home page.
2/2 tests passing, build verified.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/postcss.config.js

@@ -0,0 +1,5 @@
+module.exports = {
+  plugins: {
+    '@tailwindcss/postcss': {},
+  },
+};