refactor: restructure sidebar navigation with logical groups

Sidebar reorganized from 9 flat items to 7 items in 3 groups:

CREA: Genera, Libreria, Idee
PIANIFICA: Calendario, Programmati
GESTISCI: Personaggi

Removed from primary nav:
- Link Affiliati (secondary, move to Settings later)
- Social (setup, accessible via Settings)
- Commenti (unused, future Pro feature)

Other changes:
- /content/library route added (replaces /content/archive as primary)
- /content/archive kept as fallback route
- All links updated to point to /content/library
- "Contenuti" renamed to "Genera"
- "Pianificazione" renamed to "Calendario"
- "Schedulazione" renamed to "Programmati"
- "Nuovo piano" button removed from Dashboard
- Nav group headers with uppercase labels
- end prop on /content to avoid highlighting on /content/library

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

frontend/src/components/ContentPage.jsx

@@ -212,7 +212,7 @@ export default function ContentPage() {
           <p style={{ fontSize: '0.875rem', color: 'var(--ink-muted)', margin: 0 }}>
             Definisci un brief editoriale, scegli piattaforma e tipo, poi genera. L'AI terrà conto del tono e dei topic del personaggio selezionato.
           </p>
-          <Link to="/content/archive" style={{ fontSize: '0.8rem', color: 'var(--accent)', whiteSpace: 'nowrap', textDecoration: 'none', fontWeight: 600 }}>
+          <Link to="/content/library" style={{ fontSize: '0.8rem', color: 'var(--accent)', whiteSpace: 'nowrap', textDecoration: 'none', fontWeight: 600 }}>
             Libreria →
           </Link>
         </div>