fix: sanitize cookie expires for Playwright compatibility

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>

playwright.go

@@ -122,12 +122,17 @@ func playwrightSameSiteToSameSite(s *playwright.SameSiteAttribute) SameSite {
 }
 
 func cookieToPlaywrightOptionalCookie(cookie Cookie) playwright.OptionalCookie {
+	expires := float64(cookie.Expires.Unix())
+	if cookie.Expires.IsZero() || expires <= 0 {
+		expires = -1
+	}
+
 	oc := playwright.OptionalCookie{
 		Name:     cookie.Name,
 		Value:    cookie.Value,
 		Domain:   playwright.String(cookie.Host),
 		Path:     playwright.String(cookie.Path),
-		Expires:  playwright.Float(float64(cookie.Expires.Unix())),
+		Expires:  playwright.Float(expires),
 		Secure:   playwright.Bool(cookie.Secure),
 		HttpOnly: playwright.Bool(cookie.HttpOnly),
 	}