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>

internal/web/templates/layout.html

@@ -19,6 +19,11 @@
 <main>
 {{if .User}}{{template "nav" .}}{{end}}
 {{template "content" .}}
+{{/* The running version, on every authenticated page: it is what a backup
+     manifest is checked against on restore and the first thing to establish
+     when something behaves unexpectedly. Only for signed-in administrators —
+     the login and setup pages must not advertise it to the internet. */}}
+{{if .User}}<footer class="version">SelfPost {{.Version}}</footer>{{end}}
 </main>
 </body>
 </html>{{end}}