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>

browser_init.go

@@ -152,12 +152,11 @@ func initBrowser(opt BrowserOptions) (*browserInitResult, error) {
 		if err != nil {
 			return nil, fmt.Errorf("error getting cookies from cookie jar: %w", err)
 		}
-		pwCookies := make([]playwright.OptionalCookie, len(cookies))
-		for i, c := range cookies {
-			pwCookies[i] = cookieToPlaywrightOptionalCookie(c)
-		}
-		if err := bctx.AddCookies(pwCookies); err != nil {
-			return nil, fmt.Errorf("error adding cookies to browser: %w", err)
+		for _, c := range cookies {
+			oc := cookieToPlaywrightOptionalCookie(c)
+			if err := bctx.AddCookies([]playwright.OptionalCookie{oc}); err != nil {
+				slog.Warn("skipping invalid cookie", "name", c.Name, "host", c.Host, "error", err)
+			}
 		}
 	}