Make Seed a component

Conceptually it works better as a resource, but that's an extra layer of complexity for the upcoming automatic distribution work.

src/game/plugin.rs

@@ -96,10 +96,10 @@ impl Plugin for GamePlugin {
                 info!("Will retrieve seed from peer => {peer}");
             }
             DataSource::Seed(seed) => {
-                app.insert_resource(seed);
+                app.world_mut().spawn(seed);
             }
             DataSource::None => {
-                app.insert_resource(Seed::random());
+                app.world_mut().spawn(Seed::random());
             }
         };
     }