Add Migadu domain admin with shared lists, compaction, and safer apply.

Build the Go API and React UI for managing domain denylist/allowlist/recipient
deny and spam settings, plus shared lists that import/apply across domains with
wildcard compaction, entry verification, and Migadu-friendly list encoding
(including per-domain recipient filtering and rejection bisect on apply).

internal/migadu/client_test.go

@@ -0,0 +1,64 @@
+package migadu
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestStringListUnmarshal(t *testing.T) {
+	cases := []struct {
+		name string
+		raw  string
+		want []string
+	}{
+		{name: "null", raw: `null`, want: []string{}},
+		{name: "empty string", raw: `""`, want: []string{}},
+		{name: "csv string", raw: `"a@x.com, b@y.com"`, want: []string{"a@x.com", "b@y.com"}},
+		{name: "array", raw: `["a@x.com","b@y.com"]`, want: []string{"a@x.com", "b@y.com"}},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			var got StringList
+			if err := json.Unmarshal([]byte(tc.raw), &got); err != nil {
+				t.Fatalf("unmarshal: %v", err)
+			}
+			if len(got) != len(tc.want) {
+				t.Fatalf("len=%d want %d (%v)", len(got), len(tc.want), got)
+			}
+			for i := range tc.want {
+				if got[i] != tc.want[i] {
+					t.Fatalf("got %v want %v", got, tc.want)
+				}
+			}
+		})
+	}
+}
+
+func TestDomainUpdateNilListMarshalsAsEmptyArray(t *testing.T) {
+	var nilList []string
+	update := DomainUpdate{
+		SenderDenylist:    &nilList,
+		SenderAllowlist:   &[]string{"a@b.com", "c@d.com"},
+		RecipientDenylist: &[]string{},
+	}
+	update.normalizeListPointers()
+
+	raw, err := json.Marshal(update)
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+	var decoded map[string]json.RawMessage
+	if err := json.Unmarshal(raw, &decoded); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if string(decoded["sender_denylist"]) != `""` {
+		t.Fatalf("sender_denylist=%s want empty string", decoded["sender_denylist"])
+	}
+	if string(decoded["sender_allowlist"]) != `"a@b.com,c@d.com"` {
+		t.Fatalf("sender_allowlist=%s", decoded["sender_allowlist"])
+	}
+	if string(decoded["recipient_denylist"]) != `""` {
+		t.Fatalf("recipient_denylist=%s want empty string", decoded["recipient_denylist"])
+	}
+}