feat : add task management with kanban and backlog

Add kanban board with drag-and-drop, backlog section, task/group
drawers, DTOs, services, and i18n translations.

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

frontend/services/dto/task-effort.ts

@@ -0,0 +1,9 @@
+export type TaskEffort = {
+    id: number
+    '@id'?: string
+    label: string
+}
+
+export type TaskEffortWrite = {
+    label: string
+}