feat(nix): add UI-embedded flake package, refresh lock + docs #799

Job	Run time
changes	2s
security / security-static	34s
security / codeql-gate	37m 42s
security / license-gate	3m 38s
lint / lint	5m 18s
pr-smoke	-1s
test / test-tsan	13m 18s
test / setup-matrix	4s
test / test-windows-guards	6m 27s
test / test-unix (ubuntu-latest, gcc, g++)	15m 13s
test / test-unix (ubuntu-24.04-arm, gcc, g++)	1h 18m 2s
test / test-unix (macos-14, cc, c++)	13m 18s
test / test-windows (windows-latest, CLANG64, x86_64)	34m 51s
test / test-unix (macos-15-intel, cc, c++)	21m 19s
ci-ok	2s
	3h 49m 47s

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