feat(standalone): bottom-right version + source badge

A small fixed bottom-right overlay (VersionBadge) shown app-wide — on the home
page AND inside the loaded editor — surfacing this build's release tag and a
link to the source. The tag links to the exact commit when known (the pcbjam
commit pins the kicad + wxwidgets submodule revisions → our GPLv3
corresponding-source pointer, mirroring site Footer's BUILD_SHA), else the tag's
release page, else the repo root.

build-demo.mjs injects VITE_APP_TAG / VITE_GIT_SHA / VITE_REPO_URL (new --repo
flag, defaults to github.com/emergence-engineering/pcbjam).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/standalone/src/components/VersionBadge.tsx

@@ -0,0 +1,49 @@
+import { Github } from "lucide-react";
+import { APP_GIT_SHA, APP_TAG, REPO_URL } from "@/lib/config";
+
+/**
+ * Small bottom-right overlay showing this build's version + a link to the
+ * source. The standalone is GPLv3, so the tag links to the exact commit
+ * (corresponding-source pointer — the repo commit pins the kicad + wxwidgets
+ * submodule revisions) when known, else the tag's release page, else the repo
+ * root. Mounted app-wide (App.tsx) so it shows on the home page AND inside the
+ * loaded editor. `fixed` keeps it viewport-anchored on every route; z-20 sits
+ * under the editor's boot overlay (z-30) so it's hidden until the tool is up.
+ */
+export function VersionBadge() {
+  const tag = APP_TAG ?? "dev";
+  const versionUrl = APP_GIT_SHA
+    ? `${REPO_URL}/commit/${APP_GIT_SHA}`
+    : APP_TAG
+      ? `${REPO_URL}/releases/tag/${APP_TAG}`
+      : REPO_URL;
+  const versionTitle = APP_GIT_SHA
+    ? `commit ${APP_GIT_SHA.slice(0, 12)} — GPL corresponding source`
+    : APP_TAG
+      ? `release ${APP_TAG}`
+      : "source repository";
+
+  return (
+    <div className="fixed bottom-3 right-3 z-20 flex items-center gap-2 rounded bg-black/70 px-2.5 py-1 font-mono text-[11px] text-white/80 shadow">
+      <a
+        href={versionUrl}
+        target="_blank"
+        rel="noreferrer"
+        title={versionTitle}
+        className="hover:text-white"
+      >
+        pcbjam {tag}
+      </a>
+      <span className="text-white/30">·</span>
+      <a
+        href={REPO_URL}
+        target="_blank"
+        rel="noreferrer"
+        title="Source on GitHub"
+        className="inline-flex items-center gap-1 hover:text-white"
+      >
+        <Github size={12} /> source
+      </a>
+    </div>
+  );
+}