fix: check Cookies() error and use context-aware sleep

- playwright.go: check error from page.Context().Cookies() before
  iterating over results, preventing silent failures
- archive.go: replace time.Sleep(5s) with context-aware select using
  time.After, allowing the operation to be cancelled promptly

Closes #7, #18

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

playwright.go

@@ -138,6 +138,9 @@ func NewBrowser(ctx context.Context, opts ...BrowserOptions) (Browser, error) {
 func (b playWrightBrowser) updateCookies(_ context.Context, page playwright.Page) error {
 	if b.cookieJar != nil {
 		cookies, err := page.Context().Cookies(page.URL())
+		if err != nil {
+			return fmt.Errorf("error getting cookies from browser: %w", err)
+		}
 
 		for _, cookie := range cookies {
 			// TODO: add support for deleting cookies from the jar which are deleted in the browser