feat(mobile): Implement persistent theme selection

MobileApp/app/_layout.tsx

@@ -1,8 +1,9 @@
-import { DarkTheme, DefaultTheme, ThemeProvider } from '@react-navigation/native';
+import { DarkTheme, DefaultTheme, ThemeProvider as NavigationThemeProvider } from '@react-navigation/native';
 import { Stack } from 'expo-router';
 import { StatusBar } from 'expo-status-bar';
 import 'react-native-reanimated';
 
+import { ThemeProvider } from '@/context/ThemeContext';
 import { useColorScheme } from '@/hooks/use-color-scheme';
 
 export const unstable_settings = {
@@ -10,15 +11,23 @@ export const unstable_settings = {
 };
 
 export default function RootLayout() {
+  return (
+    <ThemeProvider>
+      <RootLayoutNav />
+    </ThemeProvider>
+  );
+}
+
+function RootLayoutNav() {
   const colorScheme = useColorScheme();
 
   return (
-    <ThemeProvider value={colorScheme === 'dark' ? DarkTheme : DefaultTheme}>
+    <NavigationThemeProvider value={colorScheme === 'dark' ? DarkTheme : DefaultTheme}>
       <Stack>
         <Stack.Screen name="(tabs)" options={{ headerShown: false }} />
         <Stack.Screen name="modal" options={{ presentation: 'modal', title: 'Modal' }} />
       </Stack>
       <StatusBar style="auto" />
-    </ThemeProvider>
+    </NavigationThemeProvider>
   );
 }