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/docs/static/api/open-api-schema.json

@@ -4391,6 +4391,10 @@
                             "type": "string",
                             "enum": ["logo", "logoAndText"]
                           },
+                          "autoHideOnScroll": {
+                            "default": false,
+                            "type": "boolean"
+                          },
                           "zones": {
                             "type": "object",
                             "properties": {