Implement Save without re-prompt and Save as (#1709)

* Implement Save without re-prompt and Save as
Fixes #1668

* Add save-as icon

* Make .excalidraw the default extension

* Only show save as button on supporting browsers

src/components/MobileMenu.tsx

@@ -84,6 +84,7 @@ export const MobileMenu = ({
               <Stack.Col gap={4}>
                 {actionManager.renderAction("loadScene")}
                 {actionManager.renderAction("saveScene")}
+                {actionManager.renderAction("saveAsScene")}
                 {exportButton}
                 {actionManager.renderAction("clearCanvas")}
                 <RoomDialog