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/routes/calendar.js

@@ -268,9 +268,7 @@ router.get('/google/callback', async (req, res) => {
     delete req.session.googleOAuthState;
 
     await googleCalendar.handleCallback(code);
-
-    // Initialen Sync im Hintergrund starten (kein await - Redirect soll sofort erfolgen)
-    googleCalendar.sync().catch((e) => log.error('Initial sync failed:', e.message));
+    await googleCalendar.sync();
 
     res.redirect('/settings?sync_ok=google');
   } catch (err) {