[FE-34] Build /admin/users page — user table with role filter, role change, and suspend actions

[mftee]

Overview

Administrators need a user management interface to view all users, filter by role, change roles, and suspend or delete accounts. Without this page, user management must be done directly in the database.

Background

File to create: frontend/app/(protected)/admin/users/page.tsx

Access: Admin role only (redirect non-admins to /dashboard)

Features:

• Searchable, paginated user table fetched from GET /api/users (admin endpoint — BE-15)
• Columns: Name, Email, Role badge, Created date, Last login, Status (Active/Suspended)
• Filters: Role dropdown, Status dropdown
• Per-row actions:
  • "Change role" dropdown (USER / ADMIN)
  • "Suspend" / "Unsuspend" button
  • "Delete" button (with confirmation modal)
• Bulk actions: select multiple users → bulk delete

Acceptance Criteria

• Page shows 403 Forbidden or redirects non-admin users
• Role change calls PATCH /api/users/:id and immediately updates the row
• Changing own role to non-admin shows a warning
• Delete requires typing the user's email in a confirmation modal
• Suspended users shown with a distinct visual indicator