fix: prevent Nouveau button overflow in editor base panel

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/CausalLab.tsx

@@ -1147,7 +1147,7 @@ function TabEditor({ initialId }: { initialId?: number | null }) {
               {templates.map(t => <option key={t.id} value={t.id}>{t.name}</option>)}
             </select>
             <button onClick={resetNew} title="Nouveau vide"
-              className="px-2 py-1 bg-slate-700 hover:bg-slate-600 rounded text-xs text-slate-300">
+              className="shrink-0 px-2 py-1 bg-slate-700 hover:bg-slate-600 rounded text-xs text-slate-300 whitespace-nowrap">
               ✕ Nouveau
             </button>
           </div>