Show the build in a footer, linked to its release or commit

The version moves from the header to a "Powered by Jarl" footer. A build
of a release tag (HEAD tagged v<VERSION>, no tracked changes; the release
workflow sets RELEASE_TAG) shows and links the release; any other build
shows and links its commit, marked -modified with uncommitted changes.
jarld version, jarl --version, /status and /auth/whoami report the same.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/web/static/assets/style.css

@@ -97,7 +97,6 @@ header.top {
 header.top .brand { display: inline-flex; align-items: center; gap: 8px; color: var(--text); text-decoration: none; }
 header.top .brand .logo { display: block; transition: transform 0.15s; }
 header.top .brand .brand-name { display: block; color: var(--text); }
-header.top .brand .brand-version { color: var(--muted); font-size: 12px; align-self: flex-end; padding-bottom: 2px; }
 header.top .brand:hover { text-decoration: none; }
 header.top .brand:hover .brand-name { color: var(--accent); }
 header.top .brand:hover .logo { transform: scale(1.06); }
@@ -403,3 +402,10 @@ button.chip:hover { background: var(--accent-soft); border-color: var(--accent);
 .swatch.selected { box-shadow: 0 0 0 2px currentColor; }
 .swatch:focus-visible { outline: 2px solid var(--text); outline-offset: 2px; }
 tr.selected td { background: var(--accent-soft); }
+
+/* the page footer with the Jarl build */
+.shell { min-height: 100vh; display: flex; flex-direction: column; }
+.shell > main { flex: 1; width: 100%; }
+footer.site { padding: 12px 16px 16px; text-align: center; font-size: 12px; color: var(--muted); }
+footer.site a { color: var(--muted); text-decoration: underline; text-decoration-color: var(--border); text-underline-offset: 2px; }
+footer.site a:hover { color: var(--accent); text-decoration-color: currentColor; }