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.go

@@ -0,0 +1,89 @@
+package main
+
+import (
+	"errors"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"time"
+)
+
+// MaildirStats are message counts and storage for one mailbox tree.
+type MaildirStats struct {
+	Messages int64     // files in cur+new of the INBOX maildir
+	Total    int64     // files in cur+new across all mailboxes (incl. subfolders)
+	Storage  int64     // total bytes across all mailboxes
+	Newest   time.Time // mtime of the newest message file
+}
+
+// statMaildir walks one user's Maildir (e.g. /var/mail/produktor.io/info)
+// and counts messages the same way doveadm reports them: every regular file
+// inside a mailbox's cur/ or new/ directory is one message; tmp/ holds
+// transient uploads and is ignored. doveadm.index.* files live next to the
+// maildir, never in cur/ or new/, so they do not pollute the count.
+func statMaildir(root string) (MaildirStats, error) {
+	var st MaildirStats
+	if _, err := os.Stat(root); err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
+			return st, nil // mailbox not on disk yet: report zero, not error
+		}
+		return st, err
+	}
+	err := filepath.WalkDir(root, func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		name := d.Name()
+		if d.IsDir() && (name == "cur" || name == "new") {
+			n, size, newest, err := countMaildirFiles(path)
+			if err != nil {
+				return err
+			}
+			st.Total += n
+			st.Storage += size
+			if newest.After(st.Newest) {
+				st.Newest = newest
+			}
+			if filepath.Dir(path) == root {
+				st.Messages += n // INBOX maildir sits directly under the user root
+			}
+			return filepath.SkipDir // cur/new contain only message files
+		}
+		return nil
+	})
+	if err != nil {
+		return st, err
+	}
+	if st.Newest.IsZero() {
+		st.Newest = time.Unix(0, 0)
+	}
+	return st, nil
+}
+
+// countMaildirFiles counts message files in one cur/ or new/ directory and
+// sums their sizes. Only regular files count (dovecot never places anything
+// else in cur/new); the newest mtime is returned for sorting by recency.
+func countMaildirFiles(dir string) (n, size int64, newest time.Time, err error) {
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		return 0, 0, time.Time{}, err
+	}
+	for _, e := range entries {
+		if e.IsDir() {
+			continue
+		}
+		info, err := e.Info()
+		if err != nil {
+			return 0, 0, time.Time{}, err
+		}
+		if !info.Mode().IsRegular() {
+			continue
+		}
+		n++
+		size += info.Size()
+		if info.ModTime().After(newest) {
+			newest = info.ModTime()
+		}
+	}
+	return n, size, newest, nil
+}