[FE-53] Build Department management page — create departments, manage members, view department assets

[yusuftomilola]

Overview

Admins need a dedicated page to create and manage departments — organisational units that group users and assets. Currently there is no frontend UI for department CRUD even though the backend endpoints exist (from BE-32 or later). Without this page, admins cannot set up the organisational structure required for scoped asset tracking.

Context

• Route: frontend/app/(dashboard)/departments/page.tsx (needs to be created)
• Add "Departments" to the sidebar navigation under an "Organisation" section
• API endpoints: GET /departments, POST /departments, PATCH /departments/:id, DELETE /departments/:id, GET /departments/:id/users, GET /departments/:id/assets
• React Query hooks: create in frontend/lib/query/hooks/useDepartments.ts

Acceptance Criteria

• Create frontend/app/(dashboard)/departments/page.tsx
• Table view with columns: Department Name, Manager (name), Member Count, Asset Count, Created Date, actions (Edit, Delete)
• "Add Department" button opens a modal with: name (required), description (optional), manager selector (user search dropdown)
• Clicking a department row opens a detail drawer with two tabs: "Members" (list of users with their roles) and "Assets" (list of department assets with status badges)
• Edit button pre-fills the modal; Delete requires confirmation and warns if the department has members
• Create React Query hooks: useDepartments(), useCreateDepartment(), useUpdateDepartment(), useDeleteDepartment(), useDepartmentUsers(id), useDepartmentAssets(id)
• Add "Departments" link to the sidebar