refactor(frontend): Root layout route

[amanape]

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

• 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
This PR refactors the root layout route (_oh.tsx) into smaller, more manageable components, each staying under 150 lines. The route has also been moved from _oh.tsx to _oh/route.tsx for better organization (refer to the Remix route folder convention for details). Since these components are specific to the layout route, they have been placed in the same directory as the route for cohesion.

Extras

• Remove ancient and unused /src/session.ts (relevant if our app is SSR, which we do not currently support)

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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:60d382c-nikolaik   --name openhands-app-60d382c   docker.all-hands.dev/all-hands-ai/openhands:60d382c

[tofarr]

🍰 - breaking up these controls makes sense