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>

CHANGELOG.md

@@ -5,6 +5,13 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ## [Unreleased]
 
+### Changed
+
+- **Overview's head says it as the mockup does** (overview-head): "Mail is
+  flowing. One thing needs attention." - or "Nothing needs attention", or
+  "Mail may not be flowing" when a check fails - and under it each check
+  that is not fine with its value ("TLS certificate: expires in 12 days.").
+
 ## [2.14.0] - 2026-10-17
 
 Password reset by e-mail, to a confirmed address. Migration 0013; schema 13.