feat: add Navbar and Hero components with Tailwind styling

Create landing page structure following 'Little Often' principle:

Components Created:
- Navbar.tsx: Logo + CTA button with sticky positioning
- Hero.tsx: Headline, sub-headline, dual CTAs, trust indicators, stats
- components/index.ts: Centralized exports

Design Features:
- Mobile-first responsive design (sm:, lg: breakpoints)
- WCAG AA compliant contrast ratios (4.5:1+)
- Accessible focus rings and aria-labels
- Modern color palette (indigo primary, slate neutrals)
- Smooth gradients and shadows

Layout Structure:
frontend/src/
├── components/
│   ├── layout/
│   │   └── Navbar.tsx
│   ├── sections/
│   │   └── Hero.tsx
│   └── index.ts
├── App.tsx: Clean integration of Navbar + Hero

Fixes Applied:
- Install @tailwindcss/postcss for Tailwind v4 compatibility
- Update postcss.config.js with new plugin
- Remove @tailwind directives from index.css (v4 style)

Build Verification:
✅ TypeScript compilation successful
✅ Tailwind CSS processing successful
✅ Production build completed (dist/ folder)

Content:
- Headline: 'Il DevOps tascabile che traduce i crash...'
- Sub-headline: Explains monitoring + AI + Telegram flow
- Primary CTA: 'Ottieni il tuo Webhook URL'
- Secondary CTA: 'Guarda la Demo'
- Stats: <5s response, 300+ models, €0.15/month

Refs: docs/frontend_landing_plan.md, .opencode/skills/frontend-ui-ux/

frontend/src/App.tsx

@@ -1,121 +1,33 @@
-import { useState } from 'react'
-import reactLogo from './assets/react.svg'
-import viteLogo from './assets/vite.svg'
-import heroImg from './assets/hero.png'
-import './App.css'
+import { Navbar, Hero } from './components';
+import './App.css';
 
 function App() {
-  const [count, setCount] = useState(0)
+  const handlePrimaryCta = () => {
+    console.log('Primary CTA clicked: Ottieni Webhook URL');
+    // TODO: Implementare logica per generazione webhook
+  };
+
+  const handleSecondaryCta = () => {
+    console.log('Secondary CTA clicked: Guarda Demo');
+    // TODO: Implementare logica per apertura demo
+  };
+
+  const handleNavbarCta = () => {
+    console.log('Navbar CTA clicked: Inizia Gratis');
+    // TODO: Scroll to form o apertura modal
+  };
 
   return (
-    <>
-      <section id="center">
-        <div className="hero">
-          <img src={heroImg} className="base" width="170" height="179" alt="" />
-          <img src={reactLogo} className="framework" alt="React logo" />
-          <img src={viteLogo} className="vite" alt="Vite logo" />
-        </div>
-        <div>
-          <h1>Get started</h1>
-          <p>
-            Edit <code>src/App.tsx</code> and save to test <code>HMR</code>
-          </p>
-        </div>
-        <button
-          className="counter"
-          onClick={() => setCount((count) => count + 1)}
-        >
-          Count is {count}
-        </button>
-      </section>
-
-      <div className="ticks"></div>
-
-      <section id="next-steps">
-        <div id="docs">
-          <svg className="icon" role="presentation" aria-hidden="true">
-            <use href="/icons.svg#documentation-icon"></use>
-          </svg>
-          <h2>Documentation</h2>
-          <p>Your questions, answered</p>
-          <ul>
-            <li>
-              <a href="https://vite.dev/" target="_blank">
-                <img className="logo" src={viteLogo} alt="" />
-                Explore Vite
-              </a>
-            </li>
-            <li>
-              <a href="https://react.dev/" target="_blank">
-                <img className="button-icon" src={reactLogo} alt="" />
-                Learn more
-              </a>
-            </li>
-          </ul>
-        </div>
-        <div id="social">
-          <svg className="icon" role="presentation" aria-hidden="true">
-            <use href="/icons.svg#social-icon"></use>
-          </svg>
-          <h2>Connect with us</h2>
-          <p>Join the Vite community</p>
-          <ul>
-            <li>
-              <a href="https://github.com/vitejs/vite" target="_blank">
-                <svg
-                  className="button-icon"
-                  role="presentation"
-                  aria-hidden="true"
-                >
-                  <use href="/icons.svg#github-icon"></use>
-                </svg>
-                GitHub
-              </a>
-            </li>
-            <li>
-              <a href="https://chat.vite.dev/" target="_blank">
-                <svg
-                  className="button-icon"
-                  role="presentation"
-                  aria-hidden="true"
-                >
-                  <use href="/icons.svg#discord-icon"></use>
-                </svg>
-                Discord
-              </a>
-            </li>
-            <li>
-              <a href="https://x.com/vite_js" target="_blank">
-                <svg
-                  className="button-icon"
-                  role="presentation"
-                  aria-hidden="true"
-                >
-                  <use href="/icons.svg#x-icon"></use>
-                </svg>
-                X.com
-              </a>
-            </li>
-            <li>
-              <a href="https://bsky.app/profile/vite.dev" target="_blank">
-                <svg
-                  className="button-icon"
-                  role="presentation"
-                  aria-hidden="true"
-                >
-                  <use href="/icons.svg#bluesky-icon"></use>
-                </svg>
-                Bluesky
-              </a>
-            </li>
-          </ul>
-        </div>
-      </section>
-
-      <div className="ticks"></div>
-      <section id="spacer"></section>
-    </>
-  )
+    <div className="min-h-screen bg-slate-50">
+      <Navbar onCtaClick={handleNavbarCta} />
+      <main>
+        <Hero
+          onPrimaryCtaClick={handlePrimaryCta}
+          onSecondaryCtaClick={handleSecondaryCta}
+        />
+      </main>
+    </div>
+  );
 }
 
-export default App
+export default App;