refactor(navbar): extrair NavbarShell e adicionar variante navbar no Button

Unifica a estrutura da navbar entre o app e a landing page via novo
componente NavbarShell. Centraliza estilos de botões da navbar na
variante `navbar` do Button, eliminando nav-styles.ts e as classes
inline duplicadas. AnimatedThemeToggler, RefreshPageButton e MobileNav
passam a aceitar prop `variant` para adaptar ao contexto.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/shared/components/ui/button.tsx

@@ -19,6 +19,8 @@ const buttonVariants = cva(
 				ghost:
 					"hover:bg-accent hover:text-accent-foreground dark:hover:bg-accent/50",
 				link: "text-primary underline-offset-4 hover:underline",
+				navbar:
+					"bg-transparent text-black/75 shadow-none hover:bg-black/10 hover:text-black focus-visible:ring-black/20",
 			},
 			size: {
 				default: "h-9 px-4 py-2 has-[>svg]:px-3",