fix: resolve findBrowser PATH scan with the platform's own path module (#155)

findBrowser() takes a platform argument so its PATH-scan behaviour can be exercised for every OS from one machine, but the scan used the top-level join/delimiter from node:path, which always follow the real host OS. Select the win32/posix path module from the platform argument instead, in both the implementation and the test that carried the same bug.

The only production call site passes no platform, so process.platform always won and auth login-auto was never affected. What this fixes is the cross-platform simulation: the suite failed on a Windows host, blocking the mandatory pre-commit hook. The test now composes its expected value with win32.join, so reverting the fix fails on Linux CI too.

Closes #154