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/dialog/dialog_windows.go

@@ -0,0 +1,80 @@
+//go:build windows
+
+// Package dialog wraps the handful of Win32 message boxes ipswap needs.
+//
+// This is raw user32 rather than a GUI toolkit on purpose: the fast path must
+// stay native and dependency-free, and a confirmation prompt is the only
+// blocking UI the tray side has.
+package dialog
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+var (
+	user32      = syscall.NewLazyDLL("user32.dll")
+	messageBoxW = user32.NewProc("MessageBoxW")
+)
+
+// MessageBoxW uType flags.
+const (
+	mbOK              = 0x00000000
+	mbOKCancel        = 0x00000001
+	mbYesNo           = 0x00000004
+	mbIconError       = 0x00000010
+	mbIconQuestion    = 0x00000020
+	mbIconWarning     = 0x00000030
+	mbIconInformation = 0x00000040
+	// The tray has no window to own the dialog, so without TOPMOST the box can
+	// open behind whatever is focused and look like a hang.
+	mbSetForeground = 0x00010000
+	mbTopMost       = 0x00040000
+)
+
+// MessageBoxW return values.
+const (
+	idOK  = 1
+	idYes = 6
+)
+
+func show(title, text string, flags uint32) int {
+	t, err := syscall.UTF16PtrFromString(text)
+	if err != nil {
+		return 0
+	}
+	c, err := syscall.UTF16PtrFromString(title)
+	if err != nil {
+		return 0
+	}
+	ret, _, _ := messageBoxW.Call(
+		0,
+		uintptr(unsafe.Pointer(t)),
+		uintptr(unsafe.Pointer(c)),
+		uintptr(flags|mbSetForeground|mbTopMost),
+	)
+	return int(ret)
+}
+
+// Confirm asks a yes/no question and reports whether the user said yes.
+func Confirm(title, text string) bool {
+	return show(title, text, mbYesNo|mbIconQuestion) == idYes
+}
+
+// Info shows a plain acknowledgement box.
+func Info(title, text string) {
+	show(title, text, mbOK|mbIconInformation)
+}
+
+// Error shows a failure. The caller is expected to include the raw netsh
+// output — that text is the only diagnostic the user has on site.
+func Error(title, text string) {
+	show(title, text, mbOK|mbIconError)
+}
+
+// ErrorWithRetryAsAdmin reports a failure and offers to relaunch elevated.
+// It reports whether the user accepted.
+func ErrorWithRetryAsAdmin(title, text string) bool {
+	return show(title, text+"\n\nRelaunch ipswap as administrator and try again?",
+		mbOKCancel|mbIconWarning) == idOK
+}