feat(reporting) : add Reporting dashboard front layer

LST-59 (3.1) front. Completes the Reporting module.

- New frontend/modules/reporting/ layer (auto-detected): /reporting page
  (admin middleware) consuming the 4 read-only report endpoints.
- Filters (period presets + custom dates, project, user). 4 sections (time per
  project, time per user, tasks by status, absences by type) each with a
  DataTable + a Chart.js chart (reused global registration).
- Front-side CSV export per section (useCsvExport: BOM UTF-8, ; separator).
- i18n keys (reporting.*, sidebar.admin.reporting).

nuxt build passes; /reporting routed; no route regression.

frontend/i18n/locales/fr.json

@@ -360,7 +360,52 @@
       "section": "Administration",
       "teamAbsences": "Absences équipe",
       "administration": "Administration",
-      "directory": "Répertoire"
+      "directory": "Répertoire",
+      "reporting": "Rapports"
+    }
+  },
+  "reporting": {
+    "title": "Rapports",
+    "export": "Exporter (CSV)",
+    "empty": "Aucune donnée pour cette période.",
+    "filters": {
+      "period": "Période",
+      "from": "Du",
+      "to": "Au",
+      "project": "Projet",
+      "allProjects": "Tous les projets",
+      "user": "Utilisateur",
+      "allUsers": "Tous les utilisateurs"
+    },
+    "periods": {
+      "thisMonth": "Mois courant",
+      "lastMonth": "Mois dernier",
+      "custom": "Personnalisé"
+    },
+    "sections": {
+      "timePerProject": "Temps par projet",
+      "timePerUser": "Temps par utilisateur",
+      "tasksByStatus": "Tâches par statut",
+      "absencesByType": "Absences par type"
+    },
+    "columns": {
+      "project": "Projet",
+      "code": "Code",
+      "user": "Utilisateur",
+      "status": "Statut",
+      "type": "Type",
+      "hours": "Heures",
+      "entries": "Nb saisies",
+      "count": "Nombre",
+      "days": "Jours"
+    },
+    "absenceTypes": {
+      "cp": "Congés payés",
+      "mariage_pacs": "Mariage / PACS",
+      "naissance": "Naissance",
+      "conge_parental": "Congé parental",
+      "deces": "Décès proche",
+      "maladie": "Arrêt maladie"
     }
   },
   "common": {