feat(mail-admin): read-only account view for mail.produktor.io (#74)

- admin/: Go stdlib-only HTTP viewer, lists accounts from
  config/postfix-accounts.cf with per-mailbox message counts (INBOX and
  total, same numbers doveadm reports) and storage; quota from
  dovecot-quotas.cf; Basic Auth from .env; offline tests vs fixtures
  (go test -race ./...)
- compose: mail-admin service, build admin/Dockerfile, publishes
  127.0.0.1:19945 / 172.17.0.1:19945; config and mail-data mounted :ro,
  no docker socket
- NPM proxy host 66: location /admin/ -> 172.17.0.1:19945
- README: account admin section

admin/maildir_test.go

@@ -0,0 +1,52 @@
+package main
+
+import (
+	"testing"
+)
+
+// Fixture layout (testdata/mail/produktor.io):
+//
+//	info/  cur: 2 messages, new: 1 message, .Sent/cur: 1 message
+//	ano/   cur: 1 message
+//	postmaster/ new: 1 message
+//	postman/ cur: (empty)
+//	andriy.oblivantsev/ (missing — like an account with no mail yet)
+func TestStatMaildirInfo(t *testing.T) {
+	st, err := statMaildir("testdata/mail/produktor.io/info")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if st.Messages != 3 { // INBOX: cur 2 + new 1
+		t.Errorf("Messages = %d, want 3", st.Messages)
+	}
+	if st.Total != 4 { // INBOX 3 + .Sent 1
+		t.Errorf("Total = %d, want 4", st.Total)
+	}
+	if st.Storage < 1 {
+		t.Errorf("Storage = %d, want > 0", st.Storage)
+	}
+	if st.Newest.IsZero() {
+		t.Error("Newest should be set")
+	}
+}
+
+func TestStatMaildirEmptyMailbox(t *testing.T) {
+	st, err := statMaildir("testdata/mail/produktor.io/postman")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if st.Messages != 0 || st.Total != 0 || st.Storage != 0 {
+		t.Errorf("postman should be empty, got %+v", st)
+	}
+}
+
+func TestStatMaildirMissingRoot(t *testing.T) {
+	// An account with no Maildir on disk yet must report zero, not fail.
+	st, err := statMaildir("testdata/mail/produktor.io/andriy.oblivantsev")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if st.Messages != 0 || st.Total != 0 || st.Storage != 0 {
+		t.Errorf("missing mailbox should be zero, got %+v", st)
+	}
+}