feat: show dev-<sha> version string in nav for dev builds

Production images continue to display the semver (v1.x.x). Dev images
built by CI now receive BUILD_VERSION=dev-<7-char-sha> via a Docker ARG,
and app.js skips the v prefix for non-semver strings.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

js/app.js

@@ -38,6 +38,9 @@ window.addEventListener('popstate', e => {
 
 // ── Bootstrap ─────────────────────────────────────────────────────────────────
 
-if (VERSION) document.getElementById('nav-version').textContent = `v${VERSION}`;
+if (VERSION) {
+  const label = /^\d/.test(VERSION) ? `v${VERSION}` : VERSION;
+  document.getElementById('nav-version').textContent = label;
+}
 
 handleRoute();