feat(ui) : add create task button on my-tasks and responsive kanban columns

- Add "Créer une tâche" button on my-tasks page with mandatory project selector
- TaskModal now accepts optional projects prop for project selection in create mode
- Replace fixed-width kanban columns (w-72 shrink-0) with flexible layout (min-w-36 flex-1)
- Add min-w-0 and overflow-x-hidden on default layout to properly contain content
- Kanban now adapts to screen size from 1024px to 1920px+

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/i18n/locales/fr.json

@@ -112,7 +112,8 @@
         "allEfforts": "Tous les efforts",
         "allAssignees": "Tous",
         "noTasks": "Aucune tâche",
-        "backlog": "Backlog"
+        "backlog": "Backlog",
+        "createTask": "Créer une tâche"
     },
     "dashboard": {
         "title": "Tableau de bord",