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

components/admin/AdminShell.tsx

@@ -63,7 +63,7 @@ export default function AdminShell({ children, adminName, adminEmail }: AdminShe
   };
 
   return (
-    <div className="flex min-h-screen">
+    <div className="flex min-h-screen bg-dark">
       {/* Sidebar */}
       <aside className="w-64 min-h-screen bg-dark-light border-r border-dark-border p-6 flex flex-col">
         {/* Logo */}