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_test.go

@@ -0,0 +1,27 @@
+package config
+
+import "testing"
+
+func TestResolve(t *testing.T) {
+	cases := []struct {
+		topic, host, origin, name string
+	}{
+		{"alerts", "https://ntfy.example", "https://ntfy.example", "alerts"},
+		{"alerts", "https://ntfy.example/", "https://ntfy.example", "alerts"},
+		{"https://ntfy.example/alerts", "", "https://ntfy.example", "alerts"},
+		{"ntfy.example/alerts", "", "https://ntfy.example", "alerts"},
+		{"https://ntfy.example/base/alerts", "", "https://ntfy.example/base", "alerts"},
+	}
+	for _, tc := range cases {
+		origin, name, err := Resolve(tc.topic, tc.host)
+		if err != nil {
+			t.Fatalf("%s: %v", tc.topic, err)
+		}
+		if origin != tc.origin || name != tc.name {
+			t.Fatalf("%s: got %s %s", tc.topic, origin, name)
+		}
+	}
+	if _, _, err := Resolve("alerts", ""); err == nil {
+		t.Fatal("expected missing host error")
+	}
+}