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/app.js

@@ -93,8 +93,7 @@ function layout() {
   const header = h("header", { class: "top" },
     h("a", { class: "brand", href: "#/", title: "Dashboard" },
       logo(30),
-      wordmark({ rule: false, height: 19, cls: "brand-name" }),
-      h("span", { class: "brand-version" }, me.version)),
+      wordmark({ rule: false, height: 19, cls: "brand-name" })),
     navEl,
     h("div", { class: "spacer" }),
     modeEl = h("span"),
@@ -102,7 +101,7 @@ function layout() {
     h("a", { href: "#/account", class: "user", title: (me.permissions || []).join("\n") }, me.user.username),
     me.admin ? h("a", { href: "#/users" }, "Users") : null,
     h("button", { class: "small", onclick: logout }, "Log out"));
-  mount(app, header, barEl, mainEl);
+  mount(app, h("div", { class: "shell" }, header, barEl, mainEl, footer(me.build)));
   modeHolder = modeEl;
   renderNav();
   renderBar();
@@ -110,6 +109,16 @@ function layout() {
 
 let modeHolder;
 
+// footer names the Jarl build: the release, or the commit for other
+// builds, linked to it in the repository.
+function footer(b) {
+  if (!b) return null;
+  const what = b.tag ? "release " + b.tag
+    : b.commit ? "commit " + b.commit + (b.modified ? ", with uncommitted changes" : "") : "version " + b.version;
+  return h("footer", { class: "site" }, "Powered by ",
+    h("a", { href: b.url, target: "_blank", rel: "noopener", title: "Jarl " + b.version + ", built from " + what }, "Jarl " + b.label));
+}
+
 function renderNav() {
   mount(navEl, navItems.map(([key, href, label]) => h("a", { href, class: current.nav === key ? "active" : "" }, label)));
 }