Summary

• make the backend test gate explicit by switching the documented and CI test command to zig build test -Dembed-ui=false -Dbuild-ui=false --summary all
• fix the clean-worktree failure mode where zig build test -Dbuild-ui=false still expected prebuilt embedded UI assets because UI embedding remained enabled
• update the build panic text so contributors are told about the backend-only -Dembed-ui=false path instead of being directed only toward rebuilding UI assets
• clarify in README.md that npm is required for UI-embedding builds, while backend-only tests can run without UI assets

Validation

• rm -rf ui/node_modules ui/build .generated_ui_assets.zig && zig build test -Dembed-ui=false -Dbuild-ui=false --summary all
• npm --prefix ui ci --no-audit --no-fund
• npm --prefix ui run build
• bash tests/test_e2e.sh
• zig fmt --check src/
• git diff --check

Notes

• this PR intentionally does not change the release build path; embedded UI remains the default for normal builds
• the fix is deliberately small: it makes the backend gate explicit rather than restructuring the build graph
• a larger follow-up could revisit whether build.zig should attempt to bootstrap npm at all, but that is outside the scope of this PR