Fix: Rename AdminController to PageController to match route

- Renamed AdminController to PageController
  - Route name is 'page#index' so controller must be 'PageController'
  - Fixes Could not resolve PageController error
- Updated routes.php to use PageController
- Updated info.xml navigation to point to PageController
- Removed old AdminController.php
- Improved HTML output with better styling and next steps

This was the root cause: Controller class name must match route name.
Route: page#index → Controller: PageController

App routing should now work correctly!

analyticshub/appinfo/routes.php

@@ -10,7 +10,7 @@ namespace OCA\AnalyticsHub;
 
 return [
     'routes' => [
-        // Admin route - use root path instead of /admin
+        // Admin route - use root path
         [
             'name' => 'page#index',
             'url' => '/',