panel-redesign: Help with seventeen topics; every box links to its own

Follows the design commit before it. Written by Sonnet, reviewed by
Opus.

  - view/help.go, templates/help.html: the mockup's topics, texts, ids
    and menu. The page shows the groups the reader's menu shows:
    Overview and Server for the global role, Outbound and Inbound by
    reach (Inbound only with the feature on), Account for everyone;
    boxes are numbered as shown.
  - Box.Help set on the boxes the mockups mark; the inbound MX box has
    its link again.
  - TestEveryHelpLinkLandsOnATopic also fails on a topic that no page
    links to, unless it is listed as menu-only (account) - and on a
    stale exception.

Known until the change that follows: the dns, dmarc and account topics
describe the three-choice report address, which the code does not have
yet; Account's DMARC box still links to #dns.

Not verified by running: three sentences rest on Postfix behaviour
(delivery is deferred when a required TLS session cannot be made; a
milter with default_action=tempfail answers 4xx; with "any recipient"
an address the upstream refuses comes back as a bounce).

Evidence
  - docs/assets/panel-redesign/evidence/help.png (1440x3600) and
    in-domain.png: headless Edge, real templates served locally with
    their fixtures - NOT from the container.
  - go test -count=1 ./internal/web/... ./cmd/panel: ok (six packages).

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

internal/web/view/account.go

@@ -172,9 +172,9 @@ func NewSettings(m Meta, retention, level1 string) *Settings {
 			Lead: Rich("What holds for the whole instance, whoever is signed in. Your own name, e-mail and DMARC report address are under ",
 				Link("/account", "Account"), "."),
 		},
-		Retention:     Box{No: "01", Title: "Log retention"},
+		Retention:     Box{No: "01", Title: "Log retention", Help: topicLink(HelpSettings, "How long rows are kept")},
 		RetentionDays: retention,
-		RateLimits:    Box{No: "02", Title: "Sending rate limits", ID: "rate-limits", End: Plain("Read-only")},
+		RateLimits:    Box{No: "02", Title: "Sending rate limits", ID: "rate-limits", End: Plain("Read-only"), Help: topicLink(HelpLimits, "Levels 1 and 2")},
 		RateFacts: []Fact{
 			{Label: "Level 1 · per client IP", Value: Plain(level1), Big: true,
 				Note: Rich("Set in ", Code(".env"), "; restart to change. Nothing below may exceed it.")},