feat: check the server on launch and remember the selected model

The chat showed 'Server offline' until the user opened Settings, and the selected model was forgotten on every restart. useAutoConnect fetches the models once after the persisted settings are restored; activeModelId is now persisted.

app/chat/_layout.tsx

@@ -1,4 +1,5 @@
 import { ModelSelector } from '@/components/chat/ModelSelector';
+import { useAutoConnect } from '@/hooks/useAutoConnect';
 import { useThemeColor } from '@/hooks/useThemeColor';
 import { useAppStore } from '@/store/useAppStore';
 import { Stack, useRouter } from 'expo-router';
@@ -10,6 +11,7 @@ export default function ChatLayout() {
     const router = useRouter();
     const clearChat = useAppStore(state => state.clearChat);
     const colors = useThemeColor();
+    useAutoConnect();
 
     const isLargeScreen = width > 768;
     const containerStyle = [