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/server_test.go

@@ -331,3 +331,18 @@ func TestUserDeleteSaysNothingOfReportAddresses(t *testing.T) {
 		pageLacks(t, "Delete user", out, `report address`, `will have no`, `The domains`)
 	}
 }
+
+// The Configuration card says where the panel sees the viewer from, and what
+// to set when it is the reverse proxy's own address.
+func TestHealthSaysWhereItSeesTheViewer(t *testing.T) {
+	out := renderSignedIn(t, "health", NewHealth(admin(), "").WithClientAddress("198.51.100.7", "172.18.0.1", true, false))
+	pageHas(t, "Health", out, `<div><dt>Your address</dt><dd class="sp-mono">198.51.100.7</dd><dd class="sp-muted">From X-Forwarded-For, which the trusted proxy 172.18.0.1 set</dd></div>`)
+	pageLacks(t, "Health", out, `TRUSTED_PROXY_CIDR`)
+
+	out = renderSignedIn(t, "health", NewHealth(admin(), "").WithClientAddress("172.18.0.1", "172.18.0.1", false, true))
+	pageHas(t, "Health", out, `The proxy&#39;s own: its X-Forwarded-For is not read`,
+		`<p class="mb-3 has-text-danger">Every visitor reaches the panel as <span class="sp-mono">172.18.0.1</span>, the reverse proxy, and they all share one sign-in limit. Set <code>TRUSTED_PROXY_CIDR</code> to <code>172.18.0.1/32</code>`)
+
+	out = renderSignedIn(t, "health", NewHealth(admin(), "").WithClientAddress("fd00::1", "fd00::1", false, true))
+	pageHas(t, "Health", out, `<code>fd00::1/128</code>`)
+}