fix: add dark backgrounds to all internal pages + RGPD cookie banner

- login, register, candidature, cgv, confidentialite, mentions-legales:
  add bg-dark class so white text is visible on dark background
- admin shell + protected layout: same fix for admin/dashboard pages
- CookieBanner: update styling to match navy/orange branding, add RGPD
  compliance text, include Accepter/Refuser buttons, link to confidentialite
- layout.tsx: import and render CookieBanner globally

https://claude.ai/code/session_01V8YAjpqRQ3bfBYsABYsEgo

app/(protected)/layout.tsx

@@ -38,7 +38,7 @@ export default async function DashboardLayout({
   }
 
   return (
-    <div className="flex min-h-screen">
+    <div className="flex min-h-screen bg-dark">
       <Sidebar user={profile} />
       <main className="flex-1 p-6 md:p-10 overflow-y-auto">{children}</main>
     </div>