feat(NcAppSidebar): add info slot

[Antreesy]

☑️ Resolves

• Allows to put custom content with info slot
• Expose var(--app-sidebar-close-button-offset) for custom content positioning (respecting the space taken by close button)
• Introduce NcAppSidebarHeader for a11y navigation

See https://github.com/nextcloud-libraries/nextcloud-vue/pull/6666/files?w=1 for non-whitespace changes
See https://github.com/nextcloud-libraries/nextcloud-vue/tree/feat/noid/appsidebar-header-empty-slot-vue2 for Vue2 branch to test with apps

🖼️ Screenshots

Quick prototype for Talk:

Clean	WIth explanation

2025-04-23_13h59_29.mp4

🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable
• 3️⃣ Backport to stable8 requested

[susnux]

technically makes sense - not sure about design.
But in general I like the idea of splitting it into a layout component and the specialized components, so 👍

[DorraJaouad]

Makes sense

[susnux]

Paddings seem to be off, also close button will not be placed consistent.
I think we need to polish the implementation a bit before pushing this to public API.

[Antreesy]

Added --app-sidebar-close-button-offset variable exposed to <header class="app-sidebar-header"> descendants:

noClose - true	noClose - false

The rest of paddings are set by default header content, and should not be enforced to custom content (like with background-image, there might be a need to fill all the space available)

As all of the new stuff, moved to main branch to settle it here first.

[Antreesy]

/backport! to stable8

[ShGKme]

Proposals:

• Rename a new slot to #info or something else to not mix with the entire sidebar content
• Hide new slot in empty state
• Add a hidden header for empty state
• For research - check if we need to replace __info block or define custom __info content
• Follow-up: fallback <NcAppSidebarHeader> name and title to <NcAppSidebar>'s corresponding values

[ShGKme]

I like it