feat: set dark mode as default, disable OS-based system theme

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

components/Providers.tsx

@@ -4,7 +4,7 @@ import { ThemeProvider } from "next-themes";
 
 export default function Providers({ children }: { children: React.ReactNode }) {
   return (
-    <ThemeProvider attribute="class" defaultTheme="system" enableSystem disableTransitionOnChange>
+    <ThemeProvider attribute="class" defaultTheme="dark" disableTransitionOnChange>
       {children}
     </ThemeProvider>
   );