feat: replace setup with init wizard + interactive config menu

- `unarr init` (alias: `unarr setup`): streamlined 3-step wizard
  (API key, download dir, daemon install). Removed method/name prompts
  — auto-configured from defaults.
- `unarr config [category]`: interactive menu with 7 categories
  (downloads, organization, notifications, device, region, connection,
  advanced). Direct access via `unarr config downloads`, etc.
- Extract shared helpers (openBrowser, expandHome, isTerminal) to
  helpers.go. Delete old setup.go and config.go.
- Update all "unarr setup" references to "unarr init" across daemon,
  doctor, status, README, install scripts.

internal/cmd/helpers.go

@@ -0,0 +1,56 @@
+package cmd
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strings"
+)
+
+// openBrowser opens a URL in the default browser.
+func openBrowser(url string) {
+	var c *exec.Cmd
+	switch runtime.GOOS {
+	case "darwin":
+		c = exec.Command("open", url)
+	case "windows":
+		c = exec.Command("rundll32", "url.dll,FileProtocolHandler", url)
+	default: // linux, freebsd
+		c = exec.Command("xdg-open", url)
+	}
+	_ = c.Start() // fire and forget; best-effort
+}
+
+// defaultDownloadDir returns a sensible default download directory.
+func defaultDownloadDir() string {
+	home, _ := os.UserHomeDir()
+	candidates := []string{
+		filepath.Join(home, "Media"),
+		filepath.Join(home, "Downloads", "unarr"),
+	}
+	for _, d := range candidates {
+		if fi, err := os.Stat(d); err == nil && fi.IsDir() {
+			return d
+		}
+	}
+	return filepath.Join(home, "Media")
+}
+
+// expandHome expands a leading ~/ to the user's home directory.
+func expandHome(path string) string {
+	if strings.HasPrefix(path, "~/") {
+		home, _ := os.UserHomeDir()
+		return filepath.Join(home, path[2:])
+	}
+	return path
+}
+
+// isTerminal checks if stdin is a terminal (not piped).
+func isTerminal() bool {
+	fi, err := os.Stdin.Stat()
+	if err != nil {
+		return false
+	}
+	return fi.Mode()&os.ModeCharDevice != 0
+}