health-setup-checks: the certificate's name, Postfix's greeting, the proxy

Decided by the owner on 2026-10-13 (in place of preflight), taken up on
2026-10-16; mockups b2dcefc, cc33890, 0feaa55 (his: the greeting on a row
of its own, the name check a tag beside Names).

- The certificate: health.Certificate keeps its names (DNS names, or the
  CN without any) and ForHost holds it to SELFPOST_HOSTNAME as a TLS
  client does - VerifyHostname over the DNS names, the CN only without
  any, as OpenSSL's X509_check_host and so a sending Postfix still take it
  (src/tls/tls_client.c:439, 513). A certificate for another name fails;
  the tag beside Names says matches / not for the hostname.
- The greeting: postconf -x -h smtpd_banner, read once at the panel's
  start like the served certificate's path (main.cf is not rewritten
  after), is what smtpd writes after "220 " (src/smtpd/smtpd.c:5979). A
  first word other than the hostname warns, with what to set. A fact may
  now take the whole row (Fact.Wide, sp-wide) and carry a tag beside its
  label (Fact.LabelTag); both on the kit page.
- The proxy: auth.Module.Seen says the address the panel counts sign-ins
  against, the peer, and whether X-Forwarded-For was read or left unread
  for an untrusted peer; the Configuration card shows "Your address" and,
  in the second case, warns that every visitor shares one sign-in limit
  and names the /32 (or /128) to put in TRUSTED_PROXY_CIDR.

Tests: TestCertificateForHost (CN-only, DNS names, a wildcard, the CN
ignored beside DNS names), TestSeenAddress, TestHealthSaysWhereItSeesTheViewer,
TestHealthGreetingAndCertificateName; e2e reads the stand's right state on
Health (matches, "220 mail.e2e.test ESMTP", a direct client). go vet + go
test (26 packages) and make e2e on the test server for this tree.

Docs: help (word for word with the mockup), guide § Overview and Health,
CHANGELOG [Unreleased] § Added, roadmap row done.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/web/view/static/panel.css

@@ -278,6 +278,11 @@ code { background: var(--sp-well); color: var(--sp-ink); border-radius: 3px; pad
 .sp-facts { display: grid; grid-template-columns: repeat(auto-fit, minmax(11rem, 1fr)); gap: 1rem 1.5rem; margin: 0; }
 .sp-facts dt { font-family: var(--sp-mono); font-size: var(--sp-fs-xs); letter-spacing: .08em; text-transform: uppercase; color: var(--sp-muted); }
 .sp-facts dd { margin: 0; overflow-wrap: anywhere; }
+/* A label may carry a tag, the verdict on its value (the certificate's
+   names against the hostname); it keeps the tag's own type. */
+.sp-facts dt .tag { margin-left: .4rem; text-transform: none; letter-spacing: 0; vertical-align: 1px; }
+/* A fact that takes the whole row: a line a server wrote, read as one. */
+.sp-facts .sp-wide { grid-column: 1 / -1; }
 .sp-facts dd.sp-big { font-size: 1.5rem; font-weight: 600; line-height: 1.2; }
 .sp-side { position: sticky; top: 1.5rem; }
 .menu-label { font-family: var(--sp-mono); letter-spacing: .1em; }