feat : add dashboard with Chart.js charts and filters

Implement the dashboard page with real data from the API:
- KPI cards (hours, active tasks, total tasks, projects)
- Charts: hours by day (line), hours by project (doughnut),
  tasks by status (doughnut), tasks by priority (bar),
  tasks by project (horizontal stacked bar)
- Filters: period (week/month), project, user
- Add chart.js and vue-chartjs dependencies
- Add dashboard sidebar icon and translations

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

frontend/i18n/locales/fr.json

@@ -111,8 +111,21 @@
         "hoursWorked": "Heures travaillées",
         "inProgress": "En cours",
         "done": "Terminé",
+        "filters": {
+            "period": "Période",
+            "project": "Projet",
+            "user": "Utilisateur",
+            "allProjects": "Tous les projets",
+            "allUsers": "Tous les utilisateurs"
+        },
+        "periods": {
+            "thisWeek": "Cette semaine",
+            "lastWeek": "Semaine dernière",
+            "thisMonth": "Ce mois",
+            "lastMonth": "Mois dernier"
+        },
         "stats": {
-            "hoursThisWeek": "Heures cette semaine",
+            "hoursPeriod": "Heures sur la période",
             "myActiveTasks": "Mes tâches actives",
             "completed": "terminée(s)",
             "totalTasks": "Tâches totales",
@@ -121,8 +134,8 @@
             "users": "utilisateur(s)"
         },
         "charts": {
-            "hoursByDay": "Heures par jour (semaine en cours)",
-            "hoursByProject": "Temps par projet (semaine en cours)",
+            "hoursByDay": "Heures par jour",
+            "hoursByProject": "Temps par projet",
             "tasksByStatus": "Tâches par statut",
             "tasksByPriority": "Tâches par priorité",
             "tasksByProject": "Tâches par projet"