Removed cookies and fixed export

This update includes a few small but useful improvements across Poptin.
We cleaned up some behind-the-scenes cookie handling while keeping all the required session settings intact.
On the billing page, free users will no longer see irrelevant Cancel plan or Card options.
We also improved contact exports: exports of up to 250 contacts now download instantly, while larger exports are processed in the background and sent by email. You’ll also see clearer export progress, duplicate clicks are prevented, and your selected filters stay in place throughout the process.