feat(studio): add reusable sidebar and app layout

[evanlowe]

Adds a complete Sidebar to the Studio component library and reuses it in AppLayout and the component gallery. The sidebar supports animated 240px/56px expansion, navigation and Admin groups, fixed history headings with independently scrolling sessions, and account and session menus.

• Share persisted dark/light theme switching through the existing component tokens, including portal menus, modals and drawers
• Extend Menu with account headers, radio groups, destructive actions and configurable icon-trigger hover feedback
• Add optional scroll-edge fading and background/icon-only hover feedback to the shared primitives
• Keep full Sidebar and AppLayout previews on the same component and sample data

Scope is limited to the component library, preview gallery and documentation; Studio business routes, the agent canvas and backend logic are unchanged.

Validation:

• Frontend unit tests: 1,160 passed
• TypeScript check passed
• Pre-commit checks and diff whitespace check passed
• Focused render and scroll-state checks passed for the shared components
• Browser visual review remains pending