test(frontend): add jsdom tests for the file manager

Sobe web/index.html + web/js/app.js de verdade em jsdom, intercepta a API
e cobre arrastar-e-soltar entre pastas, mover/renomear/excluir, filtros,
busca e preview. CI passa a rodar npm ci && npm test.

package.json

@@ -0,0 +1,12 @@
+{
+  "name": "notas-online-frontend-tests",
+  "private": true,
+  "version": "0.0.0",
+  "description": "Testes do frontend (jsdom) do notas-online",
+  "scripts": {
+    "test": "node --test frontend/tests/*.test.js"
+  },
+  "devDependencies": {
+    "jsdom": "^31.0.0"
+  }
+}