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/register/page.tsx

@@ -64,7 +64,7 @@ export default function RegisterPage() {
   };
 
   return (
-    <main className="min-h-screen flex items-center justify-center px-4 py-12">
+    <main className="min-h-screen flex items-center justify-center px-4 py-12 bg-dark">
       <div className="w-full max-w-md">
         {/* Logo */}
         <div className="text-center mb-8">