panel: show the running version in the layout footer

Nothing in the UI said which build was running, though it is the value a
backup manifest is compared against on restore and the first thing worth
knowing when the panel misbehaves — it was only in the startup log line
and `panel -version`.

Add it as a small footer in the shared layout, supplied from render()
alongside .Active so no handler has to pass it, and gated on .User: the
login and setup pages face the internet and should not advertise a
version. Tests cover both the footer and render() supplying the key,
since neither is visible from any single handler.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -5,6 +5,11 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ## [Unreleased]
 
+- panel: every authenticated page now ends with the running version
+  (`SelfPost 1.1.0`) in a small footer. It is the value a backup manifest is
+  checked against on restore, and the first thing to establish when the panel
+  behaves unexpectedly. The login and setup pages deliberately do not show it.
+
 - panel: the domain page now shows the **SPF and DMARC records it expects**,
   with host, value and a Copy button, next to the DKIM record it already
   showed — previously it only said "also configure SPF and DMARC (see the