[FE-37] Build /admin/activity page — user action timeline with user and action type filters

[mftee]

Overview

The activity tracker (BE-07) records user actions (login, upload, delete). The admin activity page shows these events as a searchable timeline — useful for reviewing what a specific user has done on the platform.

Background

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

Access: Admin role only

Features:

• Activity feed (vertical timeline) fetched from GET /api/activity (admin endpoint — requires BE-07 + controller)
• Each entry: icon per action type, actor name, action description, timestamp
• Filters:
  • User filter (search by email or name)
  • Action type filter (login, document upload, document delete, etc.)
  • Date range filter
• Pagination (infinite scroll or traditional)

Acceptance Criteria

• Timeline entries show action-appropriate icons (e.g., upload icon for DOCUMENT_UPLOAD)
• User filter narrows to a specific user's activity
• Date range filters work correctly with timezone handling
• Empty state shown when no activity matches the filter criteria
• "Today", "Last 7 days", "Last 30 days" quick date range shortcuts