fix(mobile): add root index.tsx redirect to prevent stale Hello World screen

Without a root app/index.tsx in the repo, a stale default Expo Router
scaffold file (showing "Hello World") could be picked up from a previous
build or CLI scaffolding and displayed instead of the real app.

The new index.tsx immediately redirects to /(auth)/, and the existing
AuthGuard in _layout.tsx forwards authenticated users to /(tabs)/.

Also registers the index screen in the root Stack and updates
docs/MobileApp.md with an expanded project structure and a new
troubleshooting entry.

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

mobile/app/_layout.tsx

@@ -118,6 +118,7 @@ function AuthGuard() {
 
   return (
     <Stack screenOptions={{ headerShown: false }}>
+      <Stack.Screen name="index" />
       <Stack.Screen name="(auth)" />
       <Stack.Screen name="(tabs)" />
     </Stack>