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>

app/api/invoices/[id]/pdf/route.ts

@@ -1,7 +1,7 @@
 import { createClient } from '@/lib/supabase/server'
 import { NextResponse } from 'next/server'
 import { renderToBuffer } from '@react-pdf/renderer'
-import { InvoicePDF } from '@/lib/invoice/pdf-template'
+import { InvoicePDF } from '@/lib/invoices/pdf-template'
 import type { Invoice, InvoiceItem, Customer, CompanySettings } from '@/types'
 
 export async function GET(

app/api/invoices/[id]/send/route.ts

@@ -3,7 +3,7 @@ import { NextResponse } from 'next/server'
 import { eventBus } from '@/lib/events'
 import { ensureInitialized } from '@/lib/init'
 import { renderToBuffer } from '@react-pdf/renderer'
-import { InvoicePDF } from '@/lib/invoice/pdf-template'
+import { InvoicePDF } from '@/lib/invoices/pdf-template'
 import { sendEmail, isResendConfigured } from '@/lib/email/resend'
 import {
   generateInvoiceEmailHtml,

app/api/invoices/route.ts

@@ -3,7 +3,7 @@ import { NextResponse } from 'next/server'
 import { eventBus } from '@/lib/events'
 import { ensureInitialized } from '@/lib/init'
 import type { CreateInvoiceInput, EntityType, Invoice, CreditNote } from '@/types'
-import { getVatRules, calculateVat, calculateTotal } from '@/lib/invoice/vat-rules'
+import { getVatRules, calculateVat, calculateTotal } from '@/lib/invoices/vat-rules'
 import { fetchExchangeRate, convertToSEK } from '@/lib/currency/riksbanken'
 import {
   createCreditNoteJournalEntry,

app/api/transactions/batch-match-invoices/route.ts

@@ -1,6 +1,6 @@
 import { createClient } from '@/lib/supabase/server'
 import { NextResponse } from 'next/server'
-import { getBestInvoiceMatch } from '@/lib/invoice/invoice-matching'
+import { getBestInvoiceMatch } from '@/lib/invoices/invoice-matching'
 import type { Transaction } from '@/types'
 
 /**