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/static/panel.css

@@ -63,6 +63,9 @@ td.actions { text-align: right; }
 @media (prefers-color-scheme: dark) { .code { background: #14171a !important; border-color: #2b3138 !important; } }
 h2 { font-size: 1.05rem; margin: 0 0 0.4rem; }
 .back { display: inline-block; margin-bottom: 1rem; }
+/* Build version, closing every authenticated page. Quiet on purpose: it is
+   reference material, not something to read on the way past. */
+.version { margin-top: 1.6rem; text-align: right; font-size: 0.8rem; color: #6b7280; }
 select, textarea {
   width: 100%; padding: 0.55rem 0.7rem; font-size: 1rem;
   border: 1px solid #cfd4da; border-radius: 6px; background: #fff; color: inherit;