fix: tell the user when Rancher Desktop is not installed or not running (1.8.3)

[terchris]

Found on Terje's Windows PC (urb-agents #1536). Rancher Desktop wasn't running. install.ps1 wrote its files, docker pull failed with failed to connect to the docker API at npipe:////./pipe/docker_engine, and it still printed "devcontainer-toolbox installed!".

Changes

• install.ps1, step 1, runs before anything is written. It checks three things, each with a plain message and the next action:
  • Not installed: "install Rancher Desktop from Company Portal, or ask your IT department".
  • Installed but not visible to this window: "open a new PowerShell window".
  • Not running: "Start Rancher Desktop from the Start menu, wait until it says it is ready, then run this command again".
• A failed docker pull stops without claiming success.
• The body runs in a script block, with return instead of exit. Under irm | iex, a stop no longer closes the user's window or leaks ErrorActionPreference into their session. (PLAN-fix-windows-quickstart task 2.1)
• install.sh: the same plain messages, and it names Rancher Desktop instead of Docker Desktop.
• The plan records the PC-test findings, plus new task 2.7: refuse to install into C:\Windows\System32, where an administrator PowerShell opens.
• Version 1.8.3.

Verification

• PowerShell via Get-Content install.ps1 -Raw | Invoke-Expression (as the Quick Start runs it), with stub docker:
  • no docker: the "not installed" message, nothing written
  • not running: the "not running" message, nothing written
  • pull fails: the download message, no "installed!"
  • In every case the session stays alive and the caller's ErrorActionPreference is unchanged.
• bash, with a stub docker: the not-running message, nothing written. On a Mac with Rancher Desktop installed but docker not on PATH: the "installed, but this terminal can't see it" message.
• install.ps1: 0 parse errors, 0 PSScriptAnalyzer findings apart from PSAvoidUsingWriteHost. install.sh: shellcheck-clean. npm run build passes.
• Not run on Windows yet. Terje's rerun of #1536 is next.

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