Redesign Shell sidebar collapse, refactor Composer, and add Portal primitive

[ankit-thesys]

Summary

This PR implements a comprehensive redesign of the Shell component's sidebar collapse behavior, refactors the Composer into a shared component across all shell variants, and introduces a new Portal primitive.

Sidebar Collapse Redesign

• Collapsed sidebar width reduced from 104px to 52px for a cleaner minimal state
• Removed background color and full border; replaced with right-border only for a sleeker look
• Logo transitions smoothly from 36px to 28px when collapsed
• Toggle icons changed from PanelLeft/PanelRight to ArrowLeftFromLine/ArrowRightFromLine
• SidebarHeader now accepts children prop, allowing NewChatButton to be placed inside the header for persistent visibility
• NewChatButton renders as a compact IconButton (SquarePen) when sidebar is collapsed

Composer Refactor

• Extracted inline Composer from Thread.tsx into dedicated components/Composer.tsx for Shell, CopilotShell, and BottomTray
• New action-bar layout: textarea above, submit button (right) below
• Custom scrollbar styling for all composer textareas
• Added isLoadingMessages guard to prevent submission while messages are loading
• Updated border, shadow, and spacing tokens for a more polished look

Composer textarea height fix

• Fixed textarea collapsing to 0px height after sending a message in all layouts (Shell, CopilotShell, BottomTray, DesktopWelcomeComposer)
• Root cause: useLayoutEffect set height: "auto" which collapsed the textarea inside flex containers with overflow: clip, causing scrollHeight to read 0
• Fix: use height: "0px" instead of "auto" and Math.max(scrollHeight, 24) for a minimum one-line height

DesktopWelcomeComposer Updates

• Simplified DOM structure (removed unnecessary wrapper div)
• Added aria-label for accessibility

New Components

• ThreadHeader: Desktop-only header bar for thread-level actions (e.g., Share button)
• Portal: Thin wrapper around @radix-ui/react-portal for rendering content outside the DOM hierarchy

Header Customization Props

• FullScreen: Added threadHeader (desktop-only header content) and mobileHeaderActions (mobile header right-side content) props
• Copilot: Added headerActions prop for custom content in the header right side
• BottomTray: Added headerActions prop for custom content in the header (before built-in thread list/new chat/minimize buttons)
• All props use ReactNode type for maximum flexibility
• Props flow through withChatProvider HOC via the existing generic parameter pattern

Storybook and Documentation

• All Shell stories updated: NewChatButton in SidebarHeader, ThreadHeader with Share button, MobileHeader with rightChildren
• Added StandaloneWithThreadHeader story demonstrating model selector + action buttons
• Added WithThreadHeader story in Shell stories
• Added CopilotWithHeaderActions and BottomTrayWithHeaderActions stories
• Updated OpenUIChat.mdx with new "Header Customization" section and props tables for all layouts
• Updated Shell.mdx with ThreadHeader and MobileHeader.rightChildren documentation

Test plan

• Verify sidebar collapse/expand works correctly at 52px width
• Verify NewChatButton renders as icon when collapsed, full button when expanded
• Verify logo size transitions smoothly between states
• Verify submit is disabled while messages are loading
• Verify composer textarea maintains proper height after sending a message
• Verify ThreadHeader is hidden on mobile, visible on desktop
• Verify threadHeader prop renders custom content in FullScreen desktop header
• Verify mobileHeaderActions renders in FullScreen mobile header
• Verify headerActions renders in Copilot and BottomTray headers
• Verify Portal renders content into document.body (check DOM)
• Run Storybook and verify all Shell, CopilotShell, BottomTray, and OpenUIChat stories render correctly
• Run pnpm build in packages/react-ui (verified passing)
• Run pnpm lint:check in packages/react-ui (verified: 0 errors, only pre-existing warnings)

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[abhithesys]

this is not needed

[ankit-thesys]

removed the portal component