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/style.css

@@ -522,6 +522,21 @@ a:hover {
   background: color-mix(in srgb, var(--accent) 10%, transparent);
 }
 
+/* Version badge in header */
+.version-badge {
+  display: none;
+  font-size: 10px;
+  color: var(--text-muted);
+  padding: 2px 8px;
+  border-radius: 10px;
+  background: color-mix(in srgb, var(--text-muted) 8%, transparent);
+  border: 1px solid color-mix(in srgb, var(--text-muted) 15%, transparent);
+  font-family: var(--font-mono, monospace);
+  white-space: nowrap;
+  cursor: default;
+  user-select: none;
+}
+
 .header-menu-dropdown {
   display: none;
   position: absolute;