Added start_url in manifest.json to fulfill requirements for installable app (#1955)

Co-authored-by: Vojislav Zelić <vodzo@Vojislavs-MacBook-Air.local>

src/pages/site.webmanifest.jsx

@@ -26,6 +26,7 @@ export async function getServerSideProps({ res }) {
     theme_color: themes[color][theme],
     background_color: themes[color][theme],
     display: "standalone",
+    start_url: settings.startUrl || "/",
   };
 
   res.setHeader("Content-Type", "application/manifest+json");