feat: invoice builder UI with dynamic line items and real-time totals

[0xdevcollins]

Closes #68

Summary

• InvoiceDrawer — full-screen dialog (max 540px) with scrollable body:
  • Client details (email required, name, invoice number)
  • Dynamic line items table — add/remove rows, description, qty, unit price, row total
  • Tax rate (%) + discount (fixed amount) with live-updating totals summary
  • Currency selector (17 currencies)
  • Due date picker and notes/memo textarea
  • Save as Draft and Save & Send footer actions
  • Zod validation; draft saving works without due date
• InvoicesTable — paginated table with:
  • Status badges (Draft, Sent, Viewed, Partial, Paid, Overdue, Cancelled)
  • Partial payment balance display
  • Per-row dropdown: Edit draft, Download PDF, Send to client, Delete
  • Loading skeleton (5 rows)
  • Empty state with CTA
• invoices/page.tsx — fully wired page with:
  • Status filter tabs + search bar
  • All CRUD flows via TanStack Query mutations (create, update, delete, send, PDF)
  • Pagination controls
• useInvoices hook — typed queries and mutations for all invoice API endpoints

Test plan

• Create invoice → drawer opens, fill client + line items → Save as Draft → appears in table as DRAFT
• Edit draft → change a line item → totals update in real-time → Save as Draft
• Save & Send → invoice created/updated then sent → status transitions to SENT
• Send existing draft via row action → status transitions to SENT
• Download PDF → opens Cloudinary URL in new tab
• Delete draft → confirm dialog → removed from list
• Status filter tabs filter correctly
• Search by customer email or invoice number
• Responsive at 375px mobile breakpoint

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