fix: suppress login screen flash on load when already authenticated

Adds a synchronous inline script that checks localStorage before first
paint and sets html.ns-authed, hiding #setup-screen via CSS immediately.

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

index.html

@@ -19,6 +19,7 @@
 <script type="module" src="/src/main.js"></script>
 </head>
 <body>
+<script>(function(){try{if(localStorage.getItem('ns_cfg'))document.documentElement.classList.add('ns-authed');}catch(e){}})()</script>
 
 <!-- ─── SETUP ─── -->
 <div id="setup-screen">