feat(header): add opt-in auto-hide-on-scroll for the header (#6738)

* feat(header): add opt-in auto-hide-on-scroll for the header

New per-user header preference (default off): when enabled, the
header slides away while scrolling down and reappears on scroll-up or
near the top of the page. Disabled while in board edit mode, and
skipped entirely while focus is inside the header. Only applies when
the header is set to visible - the existing hover-to-reveal floating
controls already cover the hidden-header case.

* chore(api): regenerate the checked-in OpenAPI schema

check:openapi was failing in CI - the schema on disk hadn't been
regenerated after the header preferences gained autoHideOnScroll.

Add scripts/generate-openapi-schema.mts (and a generate:openapi
package script) alongside the existing check script so this is a
one-command fix next time the schema drifts.

* docs(users): document the header studio's visibility and auto-hide options

Header customization (drag-to-arrange controls, hide/show, hover-reveal
and pin, auto-hide-on-scroll) had no documentation on the user general
settings page.

apps/nextjs/src/components/layout/header/configurable-header.module.css

@@ -1,11 +1,16 @@
 .header {
   overflow: hidden;
+  transition: transform 220ms ease;
 }
 
 .header[data-visible="false"] {
   display: none;
 }
 
+.header[data-auto-hidden="true"]:not(:focus-within) {
+  transform: translateY(-100%);
+}
+
 .desktopContent {
   width: 100%;
   height: 100%;
@@ -176,6 +181,7 @@
 }
 
 @media (prefers-reduced-motion: reduce) {
+  .header,
   .floatingControls {
     transition: none;
   }