Add Next.js 15 app skeleton with Tailwind v4 and shadcn/ui

Next.js 15 + React 19, Tailwind v4, shadcn/ui (button, card, input,
dialog), postcss config, standalone output. ESLint extended with
next/core-web-vitals via eslint-config-next flat export. node-linker=hoisted
in .npmrc to allow standalone builds on Windows (symlink restriction).
next-env.d.ts added to .gitignore.

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

.gitignore

@@ -9,6 +9,9 @@ dist/
 build/
 *.tsbuildinfo
 
+# Auto-generated by Next.js — never edit, never commit
+next-env.d.ts
+
 # Env
 .env
 .env.local