enhance(server): introduce SERVER_HOST variable

[caugner]

Summary

Fixes #2298.

Problem

Currently, it isn't possible to preview the local dev environment on a different local device, like a smartphone, because the server is only reachable locally.

Solution

Introduce a SERVER_HOST environment variable that allows accessing the server when setting it to 0.0.0.0.

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How did you test this change?

Ran SERVER_HOST=0.0.0.0 yarn start:static-server and verified I can access the server from another device on the local network.