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/quota.go

@@ -0,0 +1,30 @@
+package main
+
+import (
+	"bufio"
+	"io"
+	"strings"
+)
+
+// parseQuotas reads a docker-mailserver dovecot-quotas.cf file: one
+// "user@domain:quota=<size>" per line (see DMS docs). Returns per-user
+// quota limits as raw strings. An absent or empty file means no limits.
+func parseQuotas(r io.Reader) (map[string]string, error) {
+	quotas := make(map[string]string)
+	sc := bufio.NewScanner(r)
+	for sc.Scan() {
+		line := strings.TrimSpace(sc.Text())
+		if line == "" || strings.HasPrefix(line, "#") {
+			continue
+		}
+		email, kv, ok := strings.Cut(line, ":")
+		email = strings.TrimSpace(email)
+		if !ok || email == "" {
+			continue
+		}
+		if _, quota, found := strings.Cut(kv, "="); found {
+			quotas[email] = strings.TrimSpace(quota)
+		}
+	}
+	return quotas, sc.Err()
+}