Scaffold ipswap: tray-driven Windows IP preset switcher

Implements the design in claude.md as a building skeleton: pure Go, no cgo,
cross-compiles to a single Windows .exe from Linux.

Architecture follows the spec's deliberate split. The fast path is native —
tray icon, grouped submenus, a Win32 MessageBox showing a live before/after
diff, then netsh. The slow path is an embedded web editor served on a random
loopback port and opened in the default browser.

Two decisions worth recording:

Reads use GetAdaptersAddresses, writes use netsh. The spec left the
enumeration mechanism open; parsing `netsh show config` breaks on a
non-English Windows because the output is localised. DNS static-vs-DHCP
origin is not exposed by that API, so it comes from one registry read.

The netsh command plan is built in portable code. That puts the delete-every-
existing-address step — the one that stops secondary addresses leaking across
switches — under test without needing a Windows box.

The editor requires the session token in a header for mutations, not just the
cookie: any page in the browser can make it POST to 127.0.0.1 with the cookie
attached, but it cannot set a header. The updater refuses to install a release
that publishes no SHA256.

Not yet done: no group picker for export (the API supports it), no
single-instance guard, and internal/server/web/app.css is reconstructed from
the description in claude.md rather than the canonical apointless.css.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/desktop/desktop_windows.go

@@ -0,0 +1,100 @@
+//go:build windows
+
+// Package desktop holds the small Windows shell integrations that do not
+// belong to any one feature: opening a URL in the default browser, and the
+// HKCU Run key that starts ipswap with Windows.
+package desktop
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"syscall"
+	"unsafe"
+
+	"golang.org/x/sys/windows/registry"
+
+	"gitea.apointless.space/bsncubed/ipswap/internal/config"
+)
+
+var (
+	shell32       = syscall.NewLazyDLL("shell32.dll")
+	shellExecuteW = shell32.NewProc("ShellExecuteW")
+)
+
+const runKeyPath = `Software\Microsoft\Windows\CurrentVersion\Run`
+
+// OpenURL opens a URL in the default browser.
+//
+// ShellExecuteW rather than `cmd /c start`: no console window to suppress and
+// no quoting rules to get wrong around a URL that already contains "?" and "&".
+func OpenURL(url string) error {
+	verb, err := syscall.UTF16PtrFromString("open")
+	if err != nil {
+		return err
+	}
+	target, err := syscall.UTF16PtrFromString(url)
+	if err != nil {
+		return err
+	}
+	ret, _, callErr := shellExecuteW.Call(
+		0,
+		uintptr(unsafe.Pointer(verb)),
+		uintptr(unsafe.Pointer(target)),
+		0, 0, 1, // no args, no cwd, SW_SHOWNORMAL
+	)
+	if ret <= 32 {
+		return fmt.Errorf("could not open %s (ShellExecuteW returned %d: %v)", url, ret, callErr)
+	}
+	return nil
+}
+
+// SetRunAtLogin adds or removes ipswap from the current user's Run key.
+//
+// HKCU rather than HKLM deliberately: HKLM needs elevation, and ipswap is
+// designed to run unelevated. It also keeps the setting per-user on a shared
+// support laptop.
+func SetRunAtLogin(enabled bool) error {
+	key, _, err := registry.CreateKey(registry.CURRENT_USER, runKeyPath, registry.SET_VALUE)
+	if err != nil {
+		return fmt.Errorf("opening the Run key: %w", err)
+	}
+	defer key.Close()
+
+	if !enabled {
+		err := key.DeleteValue(config.AppName)
+		if err != nil && err != registry.ErrNotExist {
+			return fmt.Errorf("removing the Run key value: %w", err)
+		}
+		return nil
+	}
+
+	exe, err := os.Executable()
+	if err != nil {
+		return fmt.Errorf("locating the running executable: %w", err)
+	}
+	exe, err = filepath.Abs(exe)
+	if err != nil {
+		return err
+	}
+
+	// Quoted: the path routinely contains spaces (Program Files, or a user
+	// profile with a space in the name) and the Run key value is a command
+	// line, not a path.
+	if err := key.SetStringValue(config.AppName, `"`+exe+`"`); err != nil {
+		return fmt.Errorf("writing the Run key value: %w", err)
+	}
+	return nil
+}
+
+// RunAtLogin reports whether the Run key entry is present.
+func RunAtLogin() bool {
+	key, err := registry.OpenKey(registry.CURRENT_USER, runKeyPath, registry.QUERY_VALUE)
+	if err != nil {
+		return false
+	}
+	defer key.Close()
+
+	v, _, err := key.GetStringValue(config.AppName)
+	return err == nil && v != ""
+}