Fix mobile upload page - full page not modal

- Add main_class yield to app.blade.php for proper CSS targeting
- Add upload-page-only upload-page-responsive classes to create view
- Now mobile upload (/videos/create) shows as full-width page, not modal

resources/views/layouts/app.blade.php

@@ -430,7 +430,7 @@
     @include('layouts.partials.sidebar')
     
     <!-- Main Content -->
-    <main class="yt-main" id="main">
+    <main class="yt-main @yield('main_class')" id="main">
         @yield('content')
     </main>