Fix examples browser compatibility with React 19 and React Router v7

[mvaligursky]

Fixes #8211 - issues with the examples browser after dependency updates.

React 19 Fragment Fix

• Replaced fragment() calls with div wrappers using display: contents in Example.mjs, MainLayout.mjs, and ErrorBoundary.mjs
• PCUI's Container component clones children with a parent prop, which React 19 now strictly rejects on Fragments
• Using display: contents preserves layout behavior while allowing the wrapper to accept arbitrary props

Sidebar Selection Fix

• Updated SideBar component to use React Router's useLocation hook instead of the native hashchange event
• React Router v7 navigates via History API without triggering native hash events
• Created SideBarWithRouter wrapper following the same pattern as ExampleWithRouter

Build Warning Suppression

• Added onwarn handler to rollup config to suppress "use client" directive warnings from react-router v7
• These directives are for React Server Components which the examples browser doesn't use