refactor: move ConnectionQR into standard Layout with proper navigation

- Wrap /connection/qr route in ProtectedRoute with Layout
- Remove fullscreen overlay and custom close button
- Add AdminBackButton with replace prop to avoid history cycles
- Use replace navigation from Connection to QR page
- Add replace prop support to AdminBackButton component

src/pages/Connection.tsx

@@ -37,6 +37,7 @@ export default function Connection() {
 
   const handleOpenQR = useCallback(() => {
     navigate('/connection/qr', {
+      replace: true,
       state: {
         url: appConfig?.subscriptionUrl,
         hideLink: appConfig?.hideLink ?? false,