Fix compatibility check when mediaDevices is unavailable

[volksec]

What changed

• replace eval()-based browser feature checks with guarded accessors
• return false when navigator.mediaDevices is unavailable instead of throwing a TypeError
• preserve the existing compatibility error messages
• add unit coverage for missing mediaDevices and for a fully compatible browser environment

Why

checkBrowserCompatibility() checks navigator.mediaDevices.getUserMedia by evaluating the full property path. When navigator exists but mediaDevices does not (for example, in an insecure browser context), evaluating that path throws before the method can report incompatibility and return false.

This fixes #211.

Validation

• npm test -- --runInBand tests/unit/src/utilities/HiFiUtilities.unit.test.ts — 2 tests passed
• npm run build:node — passed
• git diff --check — passed

The broader unit command passed 25 tests, but two existing suites could not start locally because the native wrtc.node binary was unavailable on Windows.