chore(frontend): Migrate from Remix to React Router 7

[amanape]

End-user friendly description of the problem this fixes or functionality that this introduces

• We are not utilizing data loaders

• We are not using SSR

• React Router v7 is the next iteration of React Router v6 and Remix v2

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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Give a summary of what the PR does, explaining any non-trivial design decisions

• Replace Remix dependencies with React Router
• Update imports
• Update scripts
• Update vite config
• Update how routes are handled (no longer file-based)

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Link of any specific issues this addresses

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:6962032-nikolaik   --name openhands-app-6962032   docker.all-hands.dev/all-hands-ai/openhands:6962032