fix: validate workers parsed from qwdtt link

[timatigoogl3-code]

Fix: workers from a qwdtt link was read via bare parseInt, so workers=abc produced NaN and workers=-5 / 9999 carried garbage into the slider power. This adds parseWorkers that only accepts an integer in 9-108 (4 hashes x 27) and returns undefined otherwise. No clamping, valid workers=18 path unchanged.

Tests: 3 new cases in frontend/src/__tests__/wdttLink.test.ts (abc, -5, 9999 -> undefined). npx vitest run src/__tests__/wdttLink.test.ts -> 18/18 pass, git diff --check clean.