feat(insights): sidebar en mode overlay sur /insights — pleine largeur

AGENTS.md: 'pas la liste carnets sur /insights'
- sidebar.tsx: isImmersiveRoute sur /insights → fixed overlay au lieu de relative
- Toutes tailles d'écran: sidebar caché par défaut, s'ouvre en drawer
- Bouton hamburger visible sur desktop + mobile (pas lg:hidden)
- Insights prend 100% de la largeur → graphe + dashboard plus spacious

memento-note/app/(main)/insights/page.tsx

@@ -297,7 +297,7 @@ export default function InsightsPage() {
       <div className="p-6 sm:p-8 border-b border-border/20 flex flex-col gap-4 sm:flex-row sm:items-center sm:justify-between sticky top-0 bg-[#F9F8F6]/80 dark:bg-[#0D0D0D]/80 backdrop-blur-md z-30 shrink-0">
         <div className="flex items-center gap-4">
           <button
-            className="lg:hidden p-2 -ms-1 text-foreground hover:bg-foreground/5 rounded-lg transition-colors shrink-0 cursor-pointer focus-visible:ring-2 focus-visible:ring-ochre/50 focus-visible:outline-none"
+            className="p-2 -ms-1 text-foreground hover:bg-foreground/5 rounded-lg transition-colors shrink-0 cursor-pointer focus-visible:ring-2 focus-visible:ring-ochre/50 focus-visible:outline-none"
             onClick={() => window.dispatchEvent(new CustomEvent('open-mobile-sidebar'))}
             aria-label={t('sidebar.openNavigation') || 'Open navigation'}
           >