Upgrade React Router from v6 to v7

[jcraigk]

Summary

We're currently on react-router-dom v6.x (^6.26.2). React Router v7 has been stable since November 2024 and is actively maintained.

We've already opted into the v7 future flags (v7_startTransition and v7_relativeSplatPath) in App.jsx, which gets us partway there.

Upgrade notes

• React Router v7 merges Remix into React Router and changes the package structure
• Imports change from react-router-dom to react-router
• New concepts like framework mode are available but optional
• See the official upgrade guide: https://reactrouter.com/upgrading/v6

Steps

1. Review the upgrade guide
2. Update the dependency in package.json to v7
3. Update imports across the codebase (react-router-dom → react-router)
4. Remove the future flags from createBrowserRouter (they become defaults in v7)
5. Test thoroughly

[jcraigk]

Fixed in 7e527f3