[FE-52] Create sticky sidebar layout component

[mftee]

Overview

Build a sticky sidebar layout component that keeps the sidebar fixed while the main content scrolls. All work goes in frontend/sandbox/.

Acceptance Criteria

• Component at frontend/sandbox/components/StickySidebarLayout.tsx
• Accepts sidebar: ReactNode and children: ReactNode props
• Sidebar sticks to the top of the viewport on scroll (position: sticky)
• Sidebar scrolls independently if its content is taller than the viewport
• On mobile (< md), sidebar collapses and is accessible via a toggle button
• Main content takes remaining width
• Demo page at frontend/sandbox/workspaces/[id]/page.tsx uses it for the workspace detail layout