feat(config): load ntfy client.yml and resolve topic origins

Parse the official ntfy client configuration, apply command-line overrides, and turn short names, host/topic pairs, and absolute URLs into an origin and topic name.

internal/config/topic.go

@@ -0,0 +1,72 @@
+package config
+
+import (
+	"fmt"
+	"net/url"
+	"strings"
+)
+
+// Resolve turns a short topic, host/topic, or absolute URL into an origin and
+// a single topic name. defaultHost is used only for short names.
+func Resolve(topic, defaultHost string) (origin, name string, err error) {
+	topic = strings.TrimSpace(topic)
+	if topic == "" {
+		return "", "", fmt.Errorf("empty topic")
+	}
+	if !strings.Contains(topic, "://") && strings.Contains(topic, "/") {
+		return Resolve("https://"+topic, "")
+	}
+	if strings.Contains(topic, "://") {
+		u, err := url.Parse(topic)
+		if err != nil || u.Host == "" || (u.Scheme != "https" && u.Scheme != "http") {
+			return "", "", fmt.Errorf("invalid topic URL %q", topic)
+		}
+		path := strings.Trim(u.Path, "/")
+		if path == "" {
+			return "", "", fmt.Errorf("topic URL %q has no topic", topic)
+		}
+		base := u.Scheme + "://" + u.Host
+		if i := strings.LastIndex(path, "/"); i >= 0 {
+			base += "/" + path[:i]
+			path = path[i+1:]
+		}
+		if err := checkName(path); err != nil {
+			return "", "", err
+		}
+		return base, path, nil
+	}
+	if err := checkName(topic); err != nil {
+		return "", "", err
+	}
+	origin, err = normalizeOrigin(defaultHost)
+	if err != nil {
+		return "", "", fmt.Errorf("topic %q: %w", topic, err)
+	}
+	return origin, topic, nil
+}
+
+func checkName(name string) error {
+	if name == "" || strings.Contains(name, ",") || strings.ContainsAny(name, " \t") {
+		return fmt.Errorf("invalid topic name %q", name)
+	}
+	return nil
+}
+
+func normalizeOrigin(host string) (string, error) {
+	host = strings.TrimSpace(host)
+	if host == "" {
+		return "", fmt.Errorf("no default-host")
+	}
+	if !strings.Contains(host, "://") {
+		host = "https://" + host
+	}
+	u, err := url.Parse(strings.TrimRight(host, "/"))
+	if err != nil || u.Host == "" || (u.Scheme != "https" && u.Scheme != "http") {
+		return "", fmt.Errorf("invalid host %q", host)
+	}
+	origin := u.Scheme + "://" + u.Host
+	if path := strings.Trim(u.Path, "/"); path != "" {
+		origin += "/" + path
+	}
+	return origin, nil
+}