Fix integrated terminal PTY on Linux

[eladrave]

Summary

• install node-pty as a host-native dependency
• remove the incompatible macOS node-pty bundled in the downloaded desktop ASAR
• build and link node-pty for Nix packages
• preserve the spawn-helper executable bit for npm's prebuilt macOS package
• add a PTY smoke test that verifies resolution outside the bundled ASAR and executes a real shell through a pseudoterminal

Why

The desktop archive is downloaded from the macOS arm64 build. Its bundled node-pty native module cannot load on Linux, so opening the integrated terminal fails. Resolving node-pty from the host installation provides a native module for the actual runtime platform.

Validation

• npm run prepare
• npm run build:browser
• npm run build:server
• npm run test:pty on macOS
• clean packed npm install followed by npm run test:pty
• clean linux/amd64 Node container install followed by npm run test:pty

This PR intentionally contains no Cloud Run, GCS, IAP, SSH, container deployment, or deployment-documentation changes.