Use Playwright with system Chrome when browsers aren't installed

On a Mac where the Python playwright package exists but ~/Library/Caches/ms-playwright is empty, p.chromium.launch(channel='chrome', headless=True) works with the installed Google Chrome, with no playwright install step. It also solves SVG rasterizing when cairosvg and rsvg-convert are missing: load the SVG inline in an HTML page with a transparent body, then screenshot with omit_background=True and a matching viewport. Set device_scale_factor=2 for a double-resolution export.