feat(webapp): show running version in Mini App footer

Compute `git describe --tags --always --dirty` at server startup
and expose via unauthenticated /api/version. Render as small muted
text at the bottom of the Mini App so the running version can be
confirmed at a glance.

Once tags exist, the badge will show e.g. v0.1.0 or v0.1.0-3-gSHA.
Until then it shows the short SHA.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

webapp/app.js

@@ -767,8 +767,22 @@ function fmtWeight(w) {
   return w === Math.floor(w) ? Math.floor(w).toString() : w.toString();
 }
 
+// ── Version badge ───────────────────────────────────────────────
+async function loadVersion() {
+  try {
+    const res = await fetch(API + "/version");
+    if (!res.ok) return;
+    const data = await res.json();
+    const badge = document.getElementById("version-badge");
+    if (badge && data.version) badge.textContent = data.version;
+  } catch (e) {
+    // Silent — footer just stays empty if unreachable
+  }
+}
+
 // ── Init ────────────────────────────────────────────────────────
 async function init() {
+  loadVersion();
   if (!userId) return;
   try {
     const data = await api("GET", "/exercises");

webapp/index.html

@@ -88,6 +88,10 @@
         <p>Loading...</p>
       </div>
     </div>
+
+    <footer id="app-footer">
+      <span id="version-badge"></span>
+    </footer>
   </div>
 
   <script src="app.js"></script>

webapp/style.css

@@ -472,3 +472,15 @@ details[open] .raw-toggle::before {
   transform: translateX(-50%) translateY(0);
   opacity: 1;
 }
+
+/* ── Footer / version badge ──────────────────────────────────── */
+
+#app-footer {
+  margin-top: 24px;
+  padding: 12px 16px 20px;
+  text-align: center;
+  font-size: 11px;
+  color: var(--tg-theme-hint-color, #999);
+  opacity: 0.6;
+  font-family: ui-monospace, SFMono-Regular, Menlo, monospace;
+}