fix(electron): prefer PowerShell 7 on Windows

[givemeurhats]

Summary

• prefer PowerShell 7 (pwsh) when available on the Windows PATH
• fall back to Windows PowerShell (powershell.exe)
• avoid recursive environment resolution while selecting the shell
• add cross-platform regression tests for both Windows cases

Testing

• pnpm exec vitest run src/server/__tests__/shell-env.test.ts

[superjodash]

+1