fix: settings page defaults to Profilo tab, banner links to Provider AI

- Change default activeSection from 'piano' to 'profilo'
- Support ?tab= query param to deep-link into settings sections
- Update missing API key banner to link to /settings?tab=ai

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/ContentPage.jsx

@@ -154,7 +154,7 @@ export default function ContentPage() {
           <p style={{ fontSize: '0.85rem', color: '#78350F', margin: '0 0 0.75rem', lineHeight: 1.6 }}>
             Per generare contenuti devi prima configurare un provider AI (Claude, OpenAI, Gemini...) e inserire la tua API key.
           </p>
-          <Link to="/settings" style={{ ...btnPrimary, backgroundColor: '#F59E0B', textDecoration: 'none', fontSize: '0.82rem', padding: '0.5rem 1rem' }}>
+          <Link to="/settings?tab=ai" style={{ ...btnPrimary, backgroundColor: '#F59E0B', textDecoration: 'none', fontSize: '0.82rem', padding: '0.5rem 1rem' }}>
             Vai alle Impostazioni →
           </Link>
         </div>