Playwright requires cookie expires to be either -1 (session cookie) or
a positive unix timestamp. When a cookie has no expiry (zero time.Time),
.Unix() returns -62135596800 which Playwright rejects. Cookies with
non-positive timestamps (e.g. Cloudflare's __cf_bm) also fail.
Now treats zero time or non-positive unix timestamps as session cookies
by setting expires to -1.
Fixes#84
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>