Fix PWA screenshots config: use format instead of sizes/type

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

config/packages/pwa.yaml

@@ -31,14 +31,12 @@ pwa:
               purpose: maskable
         screenshots:
             - src: '%kernel.project_dir%/public/favicon.png'
-              sizes: 2048x2048
-              type: image/png
               label: E-Ticket - Accueil
+              format: png
             - src: '%kernel.project_dir%/public/screen.png'
-              sizes: 1564x730
-              type: image/png
               label: E-Ticket - Billetterie associative
               form_factor: wide
+              format: png
         shortcuts:
             - name: Evenements
               short_name: Events