[BootstrapAdminUi] Add a body hookable to the create/update content composition

[vvasiloi]

Create/update pages render no .page-body, so the form card sits glued to the sticky page header — the header-to-content spacing in Tabler comes from .page-body's top margin against its .page-header sibling.

This adds a body hookable (rendering .page-body) to the create/update content composition; form_error_alert and form move under it.

BC break: hook customizations targeting sylius_admin.common.{create,update}.content.form* must retarget …content.body.form* — documented in the new UPGRADE.md.

Before	After

[vvasiloi]

CI failures are unrelated: ^1.15@alpha now resolves sylius/grid-bundle to 1.16.0-RC.3 (Jul 10), whose GridBuilderInterface BC layer (conditional re-declaration + class_alias from the new Component namespace) trips DebugClassLoader during kernel boot — reproducible on unmodified main.

[loic425]

@vvasiloi Thx for your proposal, on Sylius E-commerce how is it done?
But I'm in favour of your implementation.

[vvasiloi]

@loic425 In Sylius e-commrece it's not done at all, at least not structurally. The shared sylius_admin.common.{create,update}.content composition is the same as here, and form.html.twig opens straight into .container-xl, without a .page-body:

https://github.com/Sylius/Sylius/blob/b7d3f8455ad06cf7c8b5bbb226cce898348c7222/src/Sylius/Bundle/AdminBundle/Resources/config/app/twig_hooks/common/create.yaml#L14-L31

Sylius proper fixes the visual bug via CSS in _layout.scss, by making the header sticky and zeroes .page-body's margin:

https://github.com/Sylius/Sylius/blob/b7d3f8455ad06cf7c8b5bbb226cce898348c7222/src/Sylius/Bundle/AdminBundle/Resources/assets/styles/_layout.scss#L10-L29

plus the sticky-header script that toggles .is-sticky:

https://github.com/Sylius/Sylius/blob/b7d3f8455ad06cf7c8b5bbb226cce898348c7222/src/Sylius/Bundle/AdminBundle/Resources/assets/scripts/sticky-header.js#L1-L12

So the header-to-content gap comes from the header's own padding-bottom , and Tabler's .page-body { margin-top } is deliberately zeroed so index/show pages don't double-space. Stack emits data-sticky-header in header.html.twig but ships neither the stylesheet nor the JS, so we inherit vanilla Tabler which has the sub-header gap only via .page-body's margin-top, which create/update never render. That's the actual root cause.

The only places Sylius emits .page-body on a form page are taxon and promotion coupon, and it's per-resource, not in the shared composition: they disable the shared form hookable and swap in a sections template…

https://github.com/Sylius/Sylius/blob/b7d3f8455ad06cf7c8b5bbb226cce898348c7222/src/Sylius/Bundle/AdminBundle/Resources/config/app/twig_hooks/taxon/create.yaml#L3-L8

…that opens the wrapper itself:

https://github.com/Sylius/Sylius/blob/b7d3f8455ad06cf7c8b5bbb226cce898348c7222/src/Sylius/Bundle/AdminBundle/templates/taxon/sections.html.twig#L1-L2

So the two fixes are alternatives, not complements. There are two ways forward:

• Keep this hookable, meaning no extra CSS/JS, consistent .page-body across index/create/update, at the cost of the documented BC break.
• Port _layout.scss + sticky-header.js instead, meaning no BC break, and the data-sticky-header the Stack emits would do something.

I lean towards the structural change beacuse it also makes more sense semantically.