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-group.ts

@@ -0,0 +1,17 @@
+import type { Project } from './project'
+
+export type TaskGroup = {
+    id: number
+    '@id'?: string
+    title: string
+    description: string | null
+    color: string
+    project: Project | null
+}
+
+export type TaskGroupWrite = {
+    title: string
+    description: string | null
+    color: string
+    project: string
+}