Fix ContentUnavailableView centering on Apple TV

On tvOS, ContentUnavailableView inside a Group doesn't expand to fill
available space — it sizes to content and aligns top-leading. Add
.frame(maxWidth: .infinity, maxHeight: .infinity) to all instances
so they center correctly in their parent containers.

Yattee/Views/Settings/NetworkShareDiscoverySheet.swift

@@ -81,6 +81,7 @@ struct NetworkShareDiscoverySheet: View {
                                 service.startDiscovery()
                             }
                         }
+                        .frame(maxWidth: .infinity, maxHeight: .infinity)
                     }
                 } else {
                     // Group shares by type
@@ -124,6 +125,7 @@ struct NetworkShareDiscoverySheet: View {
             } description: {
                 Text(String(localized: "discovery.unavailable.description"))
             }
+            .frame(maxWidth: .infinity, maxHeight: .infinity)
         }
     }