overview-head: the mockup's sentence, made from the cards

The owner's "1" (2026-10-17), the two shown side by side; design 9aebe1e.

overviewHead writes "Mail is flowing. Nothing needs attention." when every
card is fine, "Mail is flowing. One thing needs attention." (or "N things
need") for warnings, and "Mail may not be flowing. ..." when a card fails;
the lead names each card that is not fine, linked, with its value inside
the sentence ("TLS certificate: expires in 12 days."), separated by
semicolons. lowerFirst lowers a value's first letter unless it is an
abbreviation ("CPU 98 %"); every value the cards write is a phrase.

Tests: TestOverviewHeadSentences (the three states, two cards, an
abbreviation), the Overview page and fragment, TestFilterDownIsAMissingSocket.
go vet + go test (26 packages) and make e2e on the test server for this
tree.

Docs: guide § Overview, CHANGELOG [Unreleased].

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

docs/guide.md

@@ -455,7 +455,9 @@ assumptions stay here in `docs/guide.md`.
 ### Overview and Health
 
 **Overview** (`/overview`, global role) shows the stamp — the worst of its
-checks — and one card per check: machine (processor and memory), supervised
+checks — with a sentence beside it ("Mail is flowing. One thing needs
+attention.", "Mail may not be flowing." when a check fails) and, under it,
+each check that is not fine with its value; then one card per check: machine (processor and memory), supervised
 processes (Postfix, the panel - whose roles include the DKIM signer - and
 their helpers), TLS certificate, queue, milter sockets and reverse DNS. A
 check the panel could not run shows as a warning, never as fine. Below the cards are the outbound domain list and, with the inbound relay