Fix tip jar button width changing while purchasing

Closes #389

Tusker/Screens/Preferences/Tip Jar/TipJarView.swift

@@ -77,8 +77,12 @@ struct TipJarView: View {
                     }
                 }
                 .onPreferenceChange(ButtonWidthKey.self) { newValue in
+                    if let buttonWidth {
+                        self.buttonWidth = max(buttonWidth, newValue)
+                    } else {
                         self.buttonWidth = newValue
                     }
+                }
                 
                 if let total = getTotalTips(), total > 0 {
                     Text("You've tipped a total of \(Text(total, format: products[0].0.priceFormatStyle)) 😍")