fix: prevent deleted toolbar bookmarks from being reverted by auto-sync

Race condition: when a user deletes a bookmark while a sync is running,
performSync's storeTombstones() overwrites the tombstone created by the
onRemoved listener, causing the follow-up sync to re-add the bookmark
from cloud.

Fix: re-read current tombstones from storage before writing to preserve
any tombstones added concurrently during the sync.

Author: Preshy