A faster landing page

The public landing page got a broad performance pass. The homepage is served from a static edge cache, the hero video defers loading until it's actually in view, and heavy sections load on demand with placeholders so the page becomes interactive sooner.

We also fixed the hero swapping between its poster image and the video, which used to cause a visible jump on load. The calendar and PnL previews are lighter too, since they no longer pull in full dashboard embeds.