test(desktop): stop exporting trimmed helpers

[liugddx]

Summary

• stop exporting six CSS test helpers whose consumers were removed by test: trim low-value tests across headless and desktop #2406
• restore the desktop knip check after test(headless): remove trim residue #2410 fixed the preceding formatter failure

Validation

• biome format . passes across 1620 files
• the six reported helper exports no longer appear in knip --workspace apps/desktop output
• test(headless): remove trim residue #2410 CI passed TypeScript before reaching the knip failure