fix: map Secure field in cookie conversions and add per-cookie error handling

The Secure field was dropped in both Playwright<->internal cookie
conversion functions, causing cookies with __Secure-/__Host- prefixes
to be rejected by Chromium. Additionally, batch AddCookies meant one
invalid cookie would fail browser creation entirely.

Changes:
- Map Secure field in cookieToPlaywrightOptionalCookie and
  playwrightCookieToCookie
- Add cookies one-by-one with slog.Warn on failure instead of
  failing the entire batch
- Add unit tests for both conversion functions

Closes #75

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

playwright.go

@@ -128,6 +128,7 @@ func cookieToPlaywrightOptionalCookie(cookie Cookie) playwright.OptionalCookie {
 		Domain:   playwright.String(cookie.Host),
 		Path:     playwright.String(cookie.Path),
 		Expires:  playwright.Float(float64(cookie.Expires.Unix())),
+		Secure:   playwright.Bool(cookie.Secure),
 		HttpOnly: playwright.Bool(cookie.HttpOnly),
 	}
 	if cookie.SameSite != "" {
@@ -143,6 +144,7 @@ func playwrightCookieToCookie(cookie playwright.Cookie) Cookie {
 		Host:     cookie.Domain,
 		Path:     cookie.Path,
 		Expires:  time.Unix(int64(cookie.Expires), 0),
+		Secure:   cookie.Secure,
 		HttpOnly: cookie.HttpOnly,
 		SameSite: playwrightSameSiteToSameSite(cookie.SameSite),
 	}