fix(google-sync): skip null items and await initial sync (#93)

Null/undefined items in the Google Calendar API response caused a
TypeError on `item.status` access, silently aborting the sync while
the OAuth callback had already redirected to the success page.

- Filter null items in `upsertGoogleEvents` with an early `continue`
- Await `sync()` in the OAuth callback so errors surface as
  `sync_error=google` redirects instead of false success

Resolves #92

Co-authored-by: Ulas Kalayci <ulas.kalayci@googlemail.com>

server/services/google-calendar.js

@@ -303,10 +303,11 @@ function upsertGoogleEvents(items, calRefId = null, calColor = GOOGLE_COLOR) {
   });
 
   for (const item of items) {
+    if (!item) continue;
     try {
       insertOrUpdate(item);
     } catch (err) {
-      log.error(`Upsert error for event ${item.id}:`, err.message);
+      log.error(`Upsert error for event ${item?.id}:`, err.message);
     }
   }
 }