Desktop: add Windows build machines like Linux ones #3659

Problem

Windows machines can now take offloaded Android builds (#3640) and run stim-server service and stim-server setup --build as a per-user scheduled task (#3658), but Stim Desktop leaves them out:

• The Add Remote Machine wizard lists only macOS and Linux tailnet peers (Tailnet.peers skips OS: "windows"), so a Windows machine cannot be added there.
• The tips, the Machine page empty state and the build suggestions count only macOS and Linux peers.
• If a Windows machine is in remote.machines anyway, its row shows the Simulators enabled toggle, although Windows cannot host.

Evidence

apps/desktop/Sources/StimKit/BuildMachines.swift TailnetOS has macOS and linux only; every Linux-specific branch (isLinux) is the "builds Android only, cannot host" case that applies to Windows too. #3658 documents the Windows setup (--build only, scheduled task, no Stim Host, --path-prepend for a version manager's node.exe shim).

Cause

The Desktop work for Linux (#3639, #3647, #3653) predates Windows workers.

Fix idea

Treat Windows peers like Linux ones: Builds only, Android-only Tools, the Windows setup command in PowerShell form, the scheduled task in the preview and mirror, the Windows Tailscale download, a "Windows · Android builds only" label, Windows counted in the build tips, empty state and build suggestions, and no Simulators toggle on a Windows row. Check that the CLI's "not macOS" rules (doctor, pool, hosting) cover Windows.