Add SideSheet component, refactor TOC and AIChat to use it

[zenoachtig]

This pull request introduces a new SideSheet component and refactors the TOC and AIChat to use it.

SideSheet component

• Adds SideSheet component, responsible for rendering a component in a fixed sidebar (with optional backdrop) on the left or right of the screen. It can be controlled with an open prop (needed for AIChat) or watch for changes on the body (needed for TableOfContents).
• Refactored TableOfContents and AIChat to use SideSheet

ScrollContainer component

• Refactored TableOfContents to use ScrollContainer, removed the custom TOCScrollContainer in favour of the standard component we use elsewhere. This gives us goodies like faded edges and scroll buttons, in addition to standardising the component.
• Updated active prop on ScrollContainer to take any query selector instead of an ID.

Other layout updates

• Moved layout of TOC's innerHeader to live in one place, namely SpaceLayout, instead of splitting across two files
• Simplified HeaderMobileMenu to no longer have
• Updated layout components (CustomizationRootLayout, SiteDynamicLayout, SiteStaticLayout) to accept both a htmlClassName and bodyClassName, needed to set some sheet-related classes that only work on html or body.
• Improved support for devices with notches by adding safe-area-insets and viewportFit: cover to the viewport meta tag.
• Simplified HeaderMobileMenu button by removing scroll tracking and state management now it's always occluded by a sidesheet.
• Fixed minor UI issues in AnnouncementBanner when using safe-area-insets

[linear]

RND-7803 Better support for docs assistant on smaller screens

RND-7450 Mobile TOC + Chat layout

[changeset-bot]

🦋 Changeset detected

Latest commit: 8ee84f1

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
gitbook	Patch

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[github-actions]

Summary of the deployments:

Version	URL	Status
Vercel	https://gitbook-v2-6vx8slvdw-gitbook.vercel.app	✅
Cloudflare	https://aaa025e8-gitbook-open-v2-preview.gitbook.workers.dev	✅

Test content

Site	2v	2c
GitBook	https://gitbook-v2-6vx8slvdw-gitbook.vercel.app/url/gitbook.com/docs	https://aaa025e8-gitbook-open-v2-preview.gitbook.workers.dev/url/gitbook.com/docs
E2E	https://gitbook-v2-6vx8slvdw-gitbook.vercel.app/url/gitbook.gitbook.io/test-gitbook-open	https://aaa025e8-gitbook-open-v2-preview.gitbook.workers.dev/url/gitbook.gitbook.io/test-gitbook-open

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Build	Status	Details	Updated (UTC)
customers-v2 (Inspect)	👍 Changes approved	78 changed	Dec 12, 2025, 11:00 AM
v2-cloudflare (Inspect)	👍 Changes approved	330 changed	Dec 12, 2025, 11:04 AM
v2-vercel (Inspect)	👍 Changes approved	340 changed	Dec 12, 2025, 11:05 AM