[FE-08] Create invoice list page with download button

[yusuftomilola]

Overview

Build an invoice list page showing the current user's invoices with a PDF download button for each. All work goes in frontend/sandbox/.

Acceptance Criteria

• Page at frontend/sandbox/invoices/page.tsx
• Table columns: Invoice #, Date, Amount, Status, Actions
• Status shown as a colored badge (paid = green, pending = yellow, overdue = red)
• Download button triggers a PDF download from the API
• Pagination controls (10 per page)
• Empty state shown when no invoices exist
• Loading skeleton while data fetches
• Uses mock data if no API connected