feat(GODT-2829): Integrate new sync service

Update imap service to use the new sync service.

The new sync state is stored as simple file on disk to avoid contention
with concurrent vault writes.

internal/services/imapservice/helpers.go

@@ -18,7 +18,6 @@
 package imapservice
 
 import (
-	"context"
 	"fmt"
 	"net/mail"
 	"time"
@@ -181,11 +180,3 @@ func wantLabels(apiLabels map[string]proton.Label, labelIDs []string) []string {
 		return WantLabel(apiLabel)
 	})
 }
-
-// sleepCtx sleeps for the given duration, or until the context is canceled.
-func sleepCtx(ctx context.Context, d time.Duration) {
-	select {
-	case <-ctx.Done():
-	case <-time.After(d):
-	}
-}