refactor: consolidate lib/invoice/ into lib/invoices/

Merge the singular lib/invoice/ directory into the plural lib/invoices/
to align with the codebase convention (transactions/, extensions/, reports/, etc.).
Updates all import paths and CLAUDE.md architecture docs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

components/invoices/InvoiceReviewContent.tsx

@@ -2,7 +2,7 @@
 
 import { Badge } from '@/components/ui/badge'
 import { Separator } from '@/components/ui/separator'
-import { getVatTreatmentLabel } from '@/lib/invoice/vat-rules'
+import { getVatTreatmentLabel } from '@/lib/invoices/vat-rules'
 import { formatCurrency } from '@/lib/utils'
 import type { Customer, Currency, VatTreatment } from '@/types'