feat: Custom Tailscale client for Headscale

Changes from upstream tailscale v1.97.0:
- Renamed service/pipe/registry to 'Tailscale-Custom' for coexistence
- Default control URL: https://vpn.softs.business
- TUN adapter: WintunTunnelType='Tailscale-Custom', unique GUID
- Named pipe: Tailscale-Custom\tailscaled
- Registry: SOFTWARE\Tailscale-Custom IPN
- ProgramData/LocalAppData paths: Tailscale-Custom

New components:
- cmd/tailscale-tray: Windows system tray app with multi-profile support
- installer/TailscaleCustom.wxs: WiX v5 MSI installer

Modified files (~22 production files):
- ipn/prefs.go, paths/paths.go, paths/paths_windows.go
- cmd/tailscaled/tailscaled.go, tailscaled_windows.go
- control/controlclient/direct.go
- net/tstun/tun_windows.go
- util/winutil/winutil_windows.go
- logpolicy/logpolicy.go
- and more (see git diff for full list)

envknob/envknob.go

@@ -607,7 +607,7 @@ func getPlatformEnvFiles() []string {
 	switch runtime.GOOS {
 	case "windows":
 		return []string{
-			filepath.Join(os.Getenv("ProgramData"), "Tailscale", "tailscaled-env.txt"),
+			filepath.Join(os.Getenv("ProgramData"), "Tailscale-Custom", "tailscaled-env.txt"),
 		}
 	case "linux":
 		if buildfeatures.HasSynology && distro.Get() == distro.Synology {