feat: add auth check on app startup

frontend-mobile/app/_layout.tsx

@@ -1,20 +1,52 @@
+import { useEffect } from 'react';
 import { Stack } from 'expo-router';
 import { StatusBar } from 'expo-status-bar';
 import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
 import { GestureHandlerRootView } from 'react-native-gesture-handler';
+import { useAuthStore } from '../store/authStore';
 
 const queryClient = new QueryClient();
 
 export default function RootLayout() {
+  const { checkAuth } = useAuthStore();
+
+  useEffect(() => {
+    checkAuth();
+  }, []);
+
   return (
     <GestureHandlerRootView style={{ flex: 1 }}>
       <QueryClientProvider client={queryClient}>
         <Stack>
           <Stack.Screen name="(tabs)" options={{ headerShown: false }} />
-          <Stack.Screen name="medicine/[id]" options={{ title: 'Medicamento' }} />
-          <Stack.Screen name="pharmacy/[id]" options={{ title: 'Farmacia' }} />
-          <Stack.Screen name="auth/login" options={{ title: 'Iniciar Sesión' }} />
-          <Stack.Screen name="auth/register" options={{ title: 'Registrarse' }} />
+          <Stack.Screen 
+            name="medicine/[id]" 
+            options={{ 
+              title: 'Medicamento',
+              headerTintColor: '#007AFF',
+            }} 
+          />
+          <Stack.Screen 
+            name="pharmacy/[id]" 
+            options={{ 
+              title: 'Farmacia',
+              headerTintColor: '#007AFF',
+            }} 
+          />
+          <Stack.Screen 
+            name="auth/login" 
+            options={{ 
+              title: 'Iniciar Sesión',
+              headerShown: false,
+            }} 
+          />
+          <Stack.Screen 
+            name="auth/register" 
+            options={{ 
+              title: 'Registrarse',
+              headerShown: false,
+            }} 
+          />
         </Stack>
         <StatusBar style="auto" />
       </QueryClientProvider>