chore(01-01): bootstrap Next.js 16 with TypeScript, App Router, Tailwind CSS v4

- Created Next.js 16.2.6 project with App Router, src/ directory, TypeScript strict mode
- Configured Tailwind CSS v4 with postcss.config.mjs
- Simplified src/app/page.tsx to Welcome to ClientHub placeholder
- Updated layout.tsx with ClientHub metadata, Italian lang, viewport export (Next.js 16 API)
- Added .gitignore covering node_modules, .env*, .next/, build artifacts

Note: create-next-app installed Next.js 16.2.6 (latest stable) instead of 15.x — fully compatible upgrade

src/app/globals.css

@@ -0,0 +1,26 @@
+@import "tailwindcss";
+
+:root {
+  --background: #ffffff;
+  --foreground: #171717;
+}
+
+@theme inline {
+  --color-background: var(--background);
+  --color-foreground: var(--foreground);
+  --font-sans: var(--font-geist-sans);
+  --font-mono: var(--font-geist-mono);
+}
+
+@media (prefers-color-scheme: dark) {
+  :root {
+    --background: #0a0a0a;
+    --foreground: #ededed;
+  }
+}
+
+body {
+  background: var(--background);
+  color: var(--foreground);
+  font-family: Arial, Helvetica, sans-serif;
+}