fix(frontend): add skip link to LegalPage 404 and landmark fixes

Add skip link and main-content id to LegalPage 404 branch.
Add aria-labelledby on legal document sections linking to their h2.

Author: pierrick-fonquerne

src/frontend/src/pages/LegalPage.tsx

@@ -12,8 +12,14 @@ const LegalPage = ({ slug }: LegalPageProps) => {
   if (!document) {
     return (
       <div className="min-h-screen bg-dark-950 flex flex-col">
+        <a
+          href="#main-content"
+          className="sr-only focus:not-sr-only focus:absolute focus:top-4 focus:left-4 focus:z-50 focus:px-4 focus:py-2 focus:bg-gold-500 focus:text-dark-950 focus:rounded-lg focus:font-medium"
+        >
+          Aller au contenu principal
+        </a>
         <Header />
-        <main className="flex-1 flex items-center justify-center">
+        <main id="main-content" role="main" className="flex-1 flex items-center justify-center">
           <div className="text-center">
             <h1 className="font-display text-3xl text-cream-100 mb-4">Page introuvable</h1>
             <p className="text-cream-400 mb-6">Le document demandé n'existe pas.</p>
@@ -74,8 +80,8 @@ const LegalPage = ({ slug }: LegalPageProps) => {
 
           <div className="space-y-8">
             {document.sections.map((section, index) => (
-              <section key={index}>
-                <h2 className="font-display text-xl font-semibold text-gold-400 mb-3">
+              <section key={index} aria-labelledby={`legal-section-${index}`}>
+                <h2 id={`legal-section-${index}`} className="font-display text-xl font-semibold text-gold-400 mb-3">
                   {section.title}
                 </h2>
                 <div