feat: badge version dans le header + tag v2.0.0-dev

- index.html: <div class="version-badge"> dans header-right
- app.js: fetch /api/health → affiche version dans le badge
- style.css: .version-badge discret (10px, mono, muted)
- Tag: v2.0.0-dev créé → git describe retourne v2.0.0-dev

frontend/js/app.js

@@ -104,4 +104,15 @@ async function init() {
 document.addEventListener("DOMContentLoaded", () => {
   init();
   Sync.init();
+  // Show version in header
+  fetch('/api/health')
+    .then(r => r.json())
+    .then(d => {
+      const badge = document.getElementById('version-badge');
+      if (badge && d.version) {
+        badge.textContent = d.version;
+        badge.style.display = 'inline-block';
+      }
+    })
+    .catch(() => {});
 });