fix(export): pass timeout to frame.waitForLoadState() (#2578)

The timeout option was already forwarded to page.goto() and
page.waitForLoadState(), but the subsequent frame loop used Playwright's
hardcoded 30 s default. Large presentations with many slides, iframes,
or complex components could exceed 30 s, making --timeout and
export.timeout ineffective.

Fixes #2577

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>