refactor: make Header a server component to avoid auth flash on load

Move session check from client-side useSession() to server-side auth(),
so the authenticated state is known at initial render. Extract interactive
parts (ThemeToggle, UserMenu, WorkshopsDropdown, NavLinks) into small
client components.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/Providers.tsx

@@ -2,7 +2,6 @@
 
 import { SessionProvider } from 'next-auth/react';
 import { ThemeProvider } from '@/contexts/ThemeContext';
-import { Header } from '@/components/layout/Header';
 import { ReactNode } from 'react';
 
 interface ProvidersProps {
@@ -13,10 +12,7 @@ export function Providers({ children }: ProvidersProps) {
   return (
     <SessionProvider>
       <ThemeProvider>
-        <div className="min-h-screen bg-background">
-          <Header />
-          {children}
-        </div>
+        {children}
       </ThemeProvider>
     </SessionProvider>
   );