Feat: page Play — backgroundColor, playLink, play-icon + fix slug jouer

- Renommage dossier content 3_play → 3_jouer (slug /jouer)
- Champ backgroundColor (code-editor) dans game.yml + exposé en API
- playLinks simplifié en champ url simple → playLink dans l'API
- Variable CSS --background-color sur la section .play
- Bouton "Jouer" avec icône play-icon (SVG data URI dans buttons.css)
- Plugin sylvainjule/code-editor ajouté via composer

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

site/templates/play.json.php

@@ -9,10 +9,8 @@ $specificData = [
       'description'      => $game->description()->value(),
       'thumbnail'        => $game->thumbnail()->toFile()?->url(),
       'backgroundImage'  => $game->backgroundImage()->toFile()?->url(),
-      'playLinks'        => $game->playLinks()->toStructure()->map(fn($l) => [
-                              'label' => $l->label()->value(),
-                              'url'   => $l->url()->value(),
-                            ])->values(),
+      'backgroundColor'  => $game->backgroundColor()->value() ?: null,
+      'playLink'         => $game->playLinks()->value() ?: null,
     ];
   })->values()
 ];