feat: apply homepage green gradient style to login/register pages

Match homepage design with professional green gradient background:
- Dark green gradient (from-[#1a4d2e] via-[#2d5a3d] to-[#1a4d2e])
- Clean white card with strong shadow (shadow-2xl)
- Brand green color for titles (#1a4d2e)
- Full screen height for better visual impact
- Consistent with homepage branding

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/login/page.tsx

@@ -77,8 +77,8 @@ export default function LoginPage() {
   };
 
   return (
-    <div className="min-h-[calc(100vh-4rem)] flex items-center justify-center py-12 px-4">
+    <div className="min-h-screen bg-gradient-to-b from-[#1a4d2e] via-[#2d5a3d] to-[#1a4d2e] flex items-center justify-center py-12 px-4">
-      <Card className="w-full max-w-md p-8">
+      <Card className="w-full max-w-md p-8 bg-white shadow-2xl border-0">
         <div className="text-center mb-8">
           <div className="flex justify-center mb-6">
             <Image
@@ -89,7 +89,7 @@ export default function LoginPage() {
               priority
             />
           </div>
-          <h1 className="text-3xl font-bold text-gray-900 mb-2">Connexion</h1>
+          <h1 className="text-3xl font-bold text-[#1a4d2e] mb-2">Connexion</h1>
           <p className="text-gray-600">
             Connectez-vous pour participer au jeu-concours
           </p>

app/register/page.tsx

@@ -35,8 +35,8 @@ export default function RegisterPage() {
   };
 
   return (
-    <div className="min-h-[calc(100vh-4rem)] flex items-center justify-center py-12">
+    <div className="min-h-screen bg-gradient-to-b from-[#1a4d2e] via-[#2d5a3d] to-[#1a4d2e] flex items-center justify-center py-12 px-4">
-      <Card className="w-full max-w-md p-8">
+      <Card className="w-full max-w-md p-8 bg-white shadow-2xl border-0">
         <div className="text-center mb-8">
           <div className="flex justify-center mb-6">
             <Image
@@ -47,7 +47,7 @@ export default function RegisterPage() {
               priority
             />
           </div>
-          <h1 className="text-3xl font-bold text-gray-900 mb-2">Inscription</h1>
+          <h1 className="text-3xl font-bold text-[#1a4d2e] mb-2">Inscription</h1>
           <p className="text-gray-600">
             Créez un compte pour participer au jeu-concours
           </p>