web/settings: move sub navigation into its own component

web/src/lib/subnav.ts

@@ -0,0 +1,15 @@
+import { browser } from "$app/environment";
+
+const defaultSettingsPage = () => {
+    if (browser) {
+        if (window.innerWidth <= 750) {
+            return "/settings";
+        }
+    }
+
+    return "/settings/appearance";
+}
+
+export {
+    defaultSettingsPage,
+}