Refonte UI : design system, logo, shell responsive et pages simples

- Nouveaux tokens (indigo/corail, statuts, plateformes) et mode sombre
- Logo et favicon Social Flow redessinés
- AppShell partagé : sidebar groupée (desktop) + barre de navigation basse (mobile)
- Suppression des pages mobiles dupliquées et des composants morts
- Tableau de bord, connexion, 404, calendrier, médiathèque, IA, statistiques

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017ADkrVcVFTT14hbB148HYS

client/src/components/ui/textarea.tsx

@@ -9,7 +9,7 @@ const Textarea = React.forwardRef<
   return (
     <textarea
       className={cn(
-        "flex min-h-[80px] w-full rounded-md border border-input bg-background px-3 py-2 text-base ring-offset-background placeholder:text-muted-foreground focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 disabled:cursor-not-allowed disabled:opacity-50 md:text-sm",
+        "flex min-h-[80px] w-full rounded-lg border border-input bg-card shadow-sm px-3 py-2 text-base ring-offset-background placeholder:text-muted-foreground focus-visible:outline-none focus-visible:border-primary focus-visible:ring-2 focus-visible:ring-ring/30 focus-visible:ring-offset-0 disabled:cursor-not-allowed disabled:opacity-50 md:text-sm",
         className
       )}
       ref={ref}