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.

scripts/generate-openapi-schema.mts

@@ -0,0 +1,8 @@
+import { writeFile } from "node:fs/promises";
+
+import { openApiDocument } from "../packages/api/src/open-api";
+
+const baseUrl = "http://localhost:3000";
+const generated = openApiDocument(baseUrl);
+
+await writeFile("apps/docs/static/api/open-api-schema.json", `${JSON.stringify(generated, null, 2)}\n`, "utf8");