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>

mobile/app/index.tsx

@@ -0,0 +1,17 @@
+/**
+ * Root index route – redirects to the auth flow on launch.
+ *
+ * expo-router renders this when the "/" route is matched (i.e. on cold start).
+ * Without this file, a stale default scaffold page ("Hello World") can appear
+ * if one was left behind by a previous build or Expo CLI scaffolding.
+ *
+ * The redirect targets the (auth) group; the AuthGuard in _layout.tsx will
+ * immediately forward authenticated users to (tabs).
+ */
+
+import { Redirect } from "expo-router";
+import React from "react";
+
+export default function RootIndex() {
+  return <Redirect href="/(auth)/" />;
+}