feat: invoice list page — filters, sort, CSV export, detail sheet

[0xdevcollins]

Closes #69

Summary

• InvoiceDetailSheet — right-side slide-in panel triggered by row click; shows customer info, line items, totals, notes, and quick-action buttons (Send, Download PDF, Copy Link, Cancel Invoice)
• InvoicesTable — sortable Amount / Status / Due Date column headers with chevron indicators; Cancel Invoice and Copy Link added to row dropdown; row click opens detail sheet without interfering with the actions menu
• Invoices page — date range filter (from/to), currency filter, clear-filters button, sort state wired to the API, Export CSV downloads current page as .csv, all new action handlers wired end-to-end

Also includes a bug fix for issue #66 (email PDF attachment was a Cloudinary URL that Resend could not fetch — switched to base64 content).

Acceptance criteria

• All invoice statuses render with correct badge styling
• Filters and search work correctly (status tabs, search, currency, date range)
• Pagination handles large datasets
• Export produces valid CSV
• Empty state displays on zero invoices
• Skeleton loaders shown during loading
• Sortable columns (amount, status, due date)
• Row click opens invoice detail sheet
• Quick actions: Send, Download PDF, Copy Link, Cancel

Test plan

• Create several invoices in different statuses and verify badge colours
• Filter by status, currency, date range and confirm table updates
• Click a row and verify the detail sheet slides in with correct data
• Click Export CSV and verify the download is valid
• Sort by Amount asc/desc and verify the order changes
• Cancel a non-paid invoice and verify status badge updates to Cancelled

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