Add WebUI auto-build workflow

[allozaur]

Close #17207

[ngxson]

Can we simplify this step by using git status? Example:

llama.cpp/.github/workflows/check-vendor.yml

Lines 40 to 52 in a19bd6f

	- name: Check for changes
	run: |
	set -euo pipefail
	# detect modified or untracked files
	changed=$(git status --porcelain --untracked-files=all || true)
	if [ -n "$changed" ]; then
	echo "Vendor sync modified files:"
	echo "$changed" | awk '{ print $2 }' | sed '/^$/d'
	echo "Failing because vendor files mismatch. Please update scripts/sync_vendor.py"
	exit 1
	else
	echo "Vendor files are up-to-date."
	fi

[ngxson]

While these variables are controlled by us, I think it's safer to use env instead. This will prevent any kind of injection attack in the future

Even better, put this script to a dedicated .js file, so it's more readable, while preventing accidentally injecting code in the future. Example here.