(feat) Add button to export trajectory on chat panel

[li-boxuan]

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

Add a button on chat panel that allows the user to download current trajectory

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

Here's an example of a downloaded trajectory:

trajectory-3775bdcc03e94d888885c86f221ee9c3.json

This is (logically) a pre-requisite for #6348, which is implementation-ready, but wouldn't make sense if users don't have a way to download trajectory.

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Link of any specific issues this addresses

#6049

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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:587859d-nikolaik   --name openhands-app-587859d   docker.all-hands.dev/all-hands-ai/openhands:587859d

[amanape]

Very cool stuff

[xingyaoww]

This is super cool!