Feat: transitions Play soignées — exit/enter directionnel + bg crossfade + game-preview

- Phase exiting (300ms) : featured + game-preview glissent/fondent dans slideDir
- Phase entering (350ms) : nouvel contenu entre depuis la direction opposée
- Swap currentIndex à 300ms : carousel grossit + bg crossfade simultanés
- Ajout game-preview (image preview côté droit, grid-area 3/13)
- Ajout preview dans le template JSON PHP
- Masqué sur mobile

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

site/templates/play.json.php

@@ -9,6 +9,7 @@ $specificData = [
       'description'      => $game->description()->value(),
       'thumbnail'        => $game->thumbnail()->toFile()?->url(),
       'backgroundColor'  => $game->backgroundColor()->value() ?: null,
+      'preview'          => $game->preview()->toFile()?->url(),
       'playLink'         => $game->playLink()->value() ?: null,
     ];
   })->values()