Warn when Chocolatey is used without an elevated shell

[trmxvibs]

Context

ChocoBackend sets requires_root = False because Windows has no sudo
and pkgwrap cannot elevate a running process. But Chocolatey genuinely
needs an elevated (Administrator) shell for install and remove. Today the
user only finds out when choco itself fails, with a message that doesn't
mention pkgwrap at all.

WindowsBackend (winget) doesn't have this problem — winget raises its own
UAC prompt when it needs elevation.

Proposal

Detect elevation on Windows and print a clear warning before running a
choco command that needs it:

import ctypes
ctypes.windll.shell32.IsUserAnAdmin()

The warning should say what to do ("run this from an Administrator
PowerShell or Command Prompt"), not just that something is wrong. This
should be a warning, not a hard failure — the user may have choco
configured differently.

Acceptance criteria

• A helper that reports elevation status on Windows and is a safe
  no-op elsewhere (Linux/macOS/CI runners)
• choco install/remove/upgrade warn when not elevated
• Tests cover both the elevated and non-elevated paths with the check
  mocked
• Nothing changes on non-Windows platforms

Pointers

• src/pkgwrap/backends/choco_backend.py
• src/pkgwrap/backends/brew_backend.py has a similar warning for the
  opposite problem (brew refuses to run as root) — follow that pattern

[Rahul-pamula]

if no one is working on this issue i willl good to work on this issue