Fix `/web` redirecting to `/web/web` in web UI (#13128)

Fixes #13127

app/javascript/mastodon/main.js

@@ -12,7 +12,7 @@ function main() {
   if (window.history && history.replaceState) {
     const { pathname, search, hash } = window.location;
     const path = pathname + search + hash;
-    if (!(/^\/web[$/]/).test(path)) {
+    if (!(/^\/web($|\/)/).test(path)) {
       history.replaceState(null, document.title, `/web${path}`);
     }
   }