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/config/config.go

@@ -0,0 +1,130 @@
+// Package config resolves where ipswap keeps its files and loads the settings
+// that are not presets.
+package config
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+// AppName is the single place the product name appears. It decides the data
+// directory, the log file name and the tray tooltip prefix, so renaming the app
+// is a one-line change here.
+const AppName = "ipswap"
+
+// Paths are the absolute locations of everything ipswap reads or writes.
+type Paths struct {
+	Dir     string
+	Presets string
+	Config  string
+	Log     string
+}
+
+// ResolvePaths returns the data locations under %APPDATA%\ipswap on Windows.
+//
+// os.UserConfigDir maps to %APPDATA% on Windows and to ~/.config elsewhere,
+// which keeps development on Linux from scribbling in odd places.
+func ResolvePaths() (Paths, error) {
+	base, err := os.UserConfigDir()
+	if err != nil {
+		return Paths{}, fmt.Errorf("locating the application data directory: %w", err)
+	}
+	dir := filepath.Join(base, AppName)
+	return Paths{
+		Dir:     dir,
+		Presets: filepath.Join(dir, "presets.json"),
+		Config:  filepath.Join(dir, "config.json"),
+		Log:     filepath.Join(dir, AppName+".log"),
+	}, nil
+}
+
+// MaskStyle is how subnet masks are displayed in the editor. Input always
+// accepts both spellings; this only controls what is rendered back.
+type MaskStyle string
+
+const (
+	MaskPrefix MaskStyle = "prefix" // /24
+	MaskDotted MaskStyle = "dotted" // 255.255.255.0
+)
+
+// Settings is config.json.
+type Settings struct {
+	// MaskStyle is global rather than per-preset: a per-preset display
+	// preference would be more state than the feature is worth.
+	MaskStyle MaskStyle `json:"mask_style"`
+
+	// StartWithWindows drives the HKCU Run key. Assumed default off; see the
+	// open questions at the bottom of claude.md.
+	StartWithWindows bool `json:"start_with_windows"`
+
+	// CheckUpdates gates the startup call to the Gitea releases API.
+	CheckUpdates bool `json:"check_updates"`
+
+	// UpdateRepo is the Gitea repo polled for releases, as
+	// "https://host/owner/repo". Empty disables the update checker entirely,
+	// which is the default until the host is confirmed.
+	UpdateRepo string `json:"update_repo"`
+
+	// ActivePollSeconds is how often the live adapter state is re-read to work
+	// out which preset is currently applied.
+	ActivePollSeconds int `json:"active_poll_seconds"`
+}
+
+// Default is the settings a fresh install starts with.
+func Default() Settings {
+	return Settings{
+		MaskStyle:         MaskPrefix,
+		StartWithWindows:  false,
+		CheckUpdates:      true,
+		UpdateRepo:        "",
+		ActivePollSeconds: 30,
+	}
+}
+
+// DottedMasks reports whether masks should render as dotted quads.
+func (s Settings) DottedMasks() bool { return s.MaskStyle == MaskDotted }
+
+// Load reads config.json, falling back to defaults for a missing file or any
+// field left unset. Settings are conveniences, so a corrupt config should not
+// stop the app from starting — the caller logs the error and carries on.
+func Load(path string) (Settings, error) {
+	s := Default()
+
+	b, err := os.ReadFile(path)
+	if os.IsNotExist(err) {
+		return s, nil
+	}
+	if err != nil {
+		return s, fmt.Errorf("reading %s: %w", path, err)
+	}
+
+	if err := json.Unmarshal(b, &s); err != nil {
+		return Default(), fmt.Errorf("parsing %s: %w", path, err)
+	}
+
+	// Repair anything nonsensical rather than refusing to run.
+	if s.MaskStyle != MaskPrefix && s.MaskStyle != MaskDotted {
+		s.MaskStyle = MaskPrefix
+	}
+	if s.ActivePollSeconds < 5 {
+		s.ActivePollSeconds = 30
+	}
+	return s, nil
+}
+
+// Save writes config.json.
+func (s Settings) Save(path string) error {
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		return fmt.Errorf("creating config directory: %w", err)
+	}
+	b, err := json.MarshalIndent(s, "", "  ")
+	if err != nil {
+		return fmt.Errorf("encoding settings: %w", err)
+	}
+	if err := os.WriteFile(path, append(b, '\n'), 0o644); err != nil {
+		return fmt.Errorf("writing %s: %w", path, err)
+	}
+	return nil
+}