feat(frontend): allow configurable backend host and port #228

Job	Run time
build-sandbox	25s
build-app (arm64, linux/arm64, ubuntu-24.04-arm)	4m 59s
build-app (amd64, linux/amd64, ubuntu-latest)	5m 6s
build-docreader	2m 55s
build-ui	5m 51s
merge	20s
	19m 36s

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