Gradient heroes and a header rethink

The hero block can now use a full-bleed gradient background and run at a tall viewport height, with selectable text position — so an ordinary hero can carry the drama of a cover.

Alongside it, the header was reworked: it scrolls away by default, with sticky as an opt-in, and the transparent overlay now applies to any dark or gradient hero at the top of a page — not just covers.