feat: runtime login screen — remove token from build

- Add LoginScreen component with Gateway URL + Token fields
- Store credentials in localStorage (not in bundle)
- Auto-reconnect with stored credentials on reload
- Add logout button (LogOut icon) in Header
- Remove VITE_GATEWAY_TOKEN from .env.example
- VITE_GATEWAY_WS_URL now only pre-fills the URL field
- Dark neon theme consistent with rest of app

Closes feedback item #4

src/App.tsx

@@ -3,11 +3,30 @@ import { useGateway } from './hooks/useGateway';
 import { Header } from './components/Header';
 import { Sidebar } from './components/Sidebar';
 import { Chat } from './components/Chat';
+import { LoginScreen } from './components/LoginScreen';
 
 export default function App() {
-  const { status, messages, sessions, activeSession, isGenerating, sendMessage, abort, switchSession } = useGateway();
+  const {
+    status, messages, sessions, activeSession, isGenerating,
+    sendMessage, abort, switchSession,
+    authenticated, login, logout, connectError, isConnecting,
+  } = useGateway();
   const [sidebarOpen, setSidebarOpen] = useState(false);
 
+  // Still checking stored credentials
+  if (authenticated === null) {
+    return (
+      <div className="h-dvh flex items-center justify-center bg-[#1e1e24] text-zinc-500">
+        <div className="animate-pulse text-sm">Connecting…</div>
+      </div>
+    );
+  }
+
+  // Not authenticated — show login
+  if (!authenticated) {
+    return <LoginScreen onConnect={login} error={connectError} isConnecting={isConnecting} />;
+  }
+
   return (
     <div className="h-dvh flex bg-[#1e1e24] text-zinc-300 bg-[radial-gradient(ellipse_at_top,rgba(255,255,255,0.02),transparent_50%),radial_gradient(ellipse_at_bottom_right,rgba(99,102,241,0.04),transparent_50%)]" role="application" aria-label="PinchChat">
       <Sidebar
@@ -18,7 +37,7 @@ export default function App() {
         onClose={() => setSidebarOpen(false)}
       />
       <div className="flex-1 flex flex-col min-w-0">
-        <Header status={status} sessionKey={activeSession} onToggleSidebar={() => setSidebarOpen(!sidebarOpen)} activeSessionData={sessions.find(s => s.key === activeSession)} />
+        <Header status={status} sessionKey={activeSession} onToggleSidebar={() => setSidebarOpen(!sidebarOpen)} activeSessionData={sessions.find(s => s.key === activeSession)} onLogout={logout} />
         <Chat messages={messages} isGenerating={isGenerating} status={status} onSend={sendMessage} onAbort={abort} />
       </div>
     </div>