Fix navigator is not defined error

[kardasbart]

Closes #61

While running locally (x86, linux) I get:

` ⨯ ReferenceError: navigator is not defined at getRecommendedWorkersNb (src/lib/engine/worker.ts:50:15) at (src/sections/analysis/states.ts:23:24) 48 | const maxWorkersNbFromThreads = Math.max(
49 | 1,

50 | Math.round(navigator.hardwareConcurrency - 4), | ^ 51 | Math.floor((navigator.hardwareConcurrency * 2) / 3) 52 | );
53 | { `

This PR solves this issue.