[FE-30] Add Excel and PDF export buttons to reports page

[yusuftomilola]

Overview

Add Excel and PDF export buttons to the reports page inside frontend/contrib/.

Location

All work must be inside frontend/contrib/app/(dashboard)/reports/.

Acceptance Criteria

• "Export Excel" button calls GET /reports/export/excel and triggers browser file download (.xlsx)
• "Export PDF" button calls GET /reports/export/pdf and triggers browser file download (.pdf)
• Both buttons show a loading spinner while the file is being generated
• Errors shown via toast notification
• Buttons placed prominently in the page header