feat: dark/light theming system + design tokens (OMC design system port)

- Restructure design tokens into :root/.dark raw vars mapped via @theme inline
- Add light/dark/system theming: useTheme hook + FOUC guard + ThemeToggle
- Keep iamcavalli palette (primary #1A463C, accent #DEF168) and Geist fonts
- Derive brand-consistent dark palette (verde-nero bg, lightened primary)
- Add scrollbar utilities (.no-scrollbar/.thin-scrollbar)
- Install tailwindcss-animate, register via @plugin (Tailwind v4)
- Mount ThemeToggle in admin sidebar footer

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/components/admin/AdminSidebar.tsx

@@ -14,6 +14,7 @@ import {
   Zap,
   FileText,
 } from "lucide-react";
+import ThemeToggle from "@/components/ThemeToggle";
 
 const NAV_ITEMS = [
   { href: "/admin",               label: "Dashboard",    icon: LayoutDashboard, exact: true },
@@ -60,8 +61,12 @@ export function AdminSidebar() {
         })}
       </nav>
 
-      {/* Logout */}
-      <div className="px-3 py-4 border-t border-white/10">
+      {/* Theme + Logout */}
+      <div className="px-3 py-4 border-t border-white/10 flex flex-col gap-2">
+        <div className="flex items-center justify-between px-3">
+          <span className="text-xs text-white/50">Tema</span>
+          <ThemeToggle />
+        </div>
         <button
           onClick={() => signOut({ callbackUrl: "/admin/login" })}
           className="flex items-center gap-3 px-3 py-2 w-full rounded-md text-sm text-white/65 hover:text-white hover:bg-white/10 transition-colors"