chore: Throw a 404 instead of returning defaults if settings does not exist

[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 way we can render the modal to enter the settings

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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:4e273c8-nikolaik   --name openhands-app-4e273c8   docker.all-hands.dev/all-hands-ai/openhands:4e273c8

[tofarr]

Clarification: Are we sure this is the desired behavior?

By using the DEFAULT_SETTINGS instead of Settings.from_config we mean that the open source environment can no longer specify default keys / models / base urls in the config.toml - these need to be filled in through the UI.

[enyst]

Clarification: Are we sure this is the desired behavior?

By using the DEFAULT_SETTINGS instead of Settings.from_config we mean that the open source environment can no longer specify default keys / models / base urls in the config.toml - these need to be filled in through the UI.

Sorry, what happened with this? It is a good point. Cc: @amanape

[amanape]

@enyst @tofarr I've rarely used the config for frontend settings-related purposes before, but we should now be able to handle all the settings typically returned from the config (for the frontend) directly in the frontend.

Do you see any cases where this might be impractical?