`next_gen_ui_demo`: display in landscape when loaded as a PWA (#1834)

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next_gen_ui_demo/web/manifest.json

@@ -6,7 +6,7 @@
     "background_color": "#0175C2",
     "theme_color": "#0175C2",
     "description": "A new Flutter project.",
-    "orientation": "portrait-primary",
+    "orientation": "landscape-primary",
     "prefer_related_applications": false,
     "icons": [
         {