fix(mobile): gate authed routes behind Stack.Protected — cold-start redbox

'/' redirected to /(tabs) unconditionally; tab screens call useApi()
which throws 'not signed in' during render — before AuthGate's effect
redirect could run. Sign-out hit the same throw.

- index redirects based on session state (api ? tabs : login)
- authed screens wrapped in Stack.Protected guard={!!api}
- found by HP-WIN11 emulator visual pass

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/mobile/src/app/_layout.tsx

@@ -19,10 +19,11 @@ function AuthGate() {
   useEffect(() => {
     if (!ready) return;
     const inLogin = segments[0] === 'login';
-    if (!api && !inLogin) router.replace('/login');
+    if (api && inLogin) router.replace('/');
-    else if (api && inLogin) router.replace('/');
   }, [ready, api, segments, router]);
 
+  if (!ready) return null;
+
   return (
     <Stack
       screenOptions={{
@@ -31,11 +32,13 @@ function AuthGate() {
         contentStyle: { backgroundColor: colors.bg },
       }}
     >
-      <Stack.Screen name="login" options={{ headerShown: false }} />
+      <Stack.Protected guard={!!api}>
         <Stack.Screen name="(tabs)" options={{ headerShown: false }} />
         <Stack.Screen name="project/[id]" options={{ title: 'Project' }} />
         <Stack.Screen name="service/[id]" options={{ title: 'Service' }} />
         <Stack.Screen name="service/logs" options={{ title: 'Logs' }} />
+      </Stack.Protected>
+      <Stack.Screen name="login" options={{ headerShown: false }} />
     </Stack>
   );
 }

app/mobile/src/app/index.tsx

@@ -1,5 +1,8 @@
 import { Redirect } from 'expo-router';
+import { useSession } from '../session';
 
 export default function Index() {
-  return <Redirect href="/(tabs)" />;
+  const { ready, api } = useSession();
+  if (!ready) return null;
+  return <Redirect href={api ? '/(tabs)' : '/login'} />;
 }