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/preset/prefix.go

@@ -0,0 +1,71 @@
+package preset
+
+import (
+	"fmt"
+	"net/netip"
+	"strconv"
+	"strings"
+)
+
+// ParsePrefix accepts every spelling of a subnet mask the editor is required to
+// take — "24", "/24" and "255.255.255.0" — and returns the prefix length.
+//
+// Only contiguous masks are accepted. 255.255.0.255 is a typo, not a netmask,
+// and letting it through would produce a netsh call that fails confusingly.
+func ParsePrefix(s string) (int, error) {
+	s = strings.TrimSpace(s)
+	if s == "" {
+		return 0, fmt.Errorf("empty mask")
+	}
+	s = strings.TrimPrefix(s, "/")
+
+	if strings.Contains(s, ".") {
+		addr, err := netip.ParseAddr(s)
+		if err != nil || !addr.Is4() {
+			return 0, fmt.Errorf("%q is not a dotted-quad netmask", s)
+		}
+		b := addr.As4()
+		mask := uint32(b[0])<<24 | uint32(b[1])<<16 | uint32(b[2])<<8 | uint32(b[3])
+
+		ones := 0
+		for ones < 32 && mask&(1<<uint(31-ones)) != 0 {
+			ones++
+		}
+		// Everything below the leading run of ones must be zero.
+		if ones < 32 && mask<<uint(ones) != 0 {
+			return 0, fmt.Errorf("%q is not a contiguous netmask", s)
+		}
+		return ones, nil
+	}
+
+	n, err := strconv.Atoi(s)
+	if err != nil {
+		return 0, fmt.Errorf("%q is not a prefix length", s)
+	}
+	if n < 0 || n > 32 {
+		return 0, fmt.Errorf("prefix /%d out of range", n)
+	}
+	return n, nil
+}
+
+// MaskString renders a prefix length as the dotted-quad netmask netsh expects
+// on the command line. netsh will not take "/24".
+func MaskString(prefix int) string {
+	if prefix < 0 || prefix > 32 {
+		prefix = 32
+	}
+	var mask uint32
+	if prefix > 0 {
+		mask = ^uint32(0) << uint(32-prefix)
+	}
+	return fmt.Sprintf("%d.%d.%d.%d", mask>>24&0xff, mask>>16&0xff, mask>>8&0xff, mask&0xff)
+}
+
+// FormatPrefix renders a prefix in whichever notation the user last chose.
+// dotted controls the style; see config.Settings.MaskStyle.
+func FormatPrefix(prefix int, dotted bool) string {
+	if dotted {
+		return MaskString(prefix)
+	}
+	return "/" + strconv.Itoa(prefix)
+}