feat: Add embedded customer info to invoices with configurable relationship

- Add customerInfo and billingAddress fields to invoice collection
- Make customer relationship optional and configurable via plugin config
- Update TypeScript types to reflect new invoice structure
- Allow disabling customer relationship with customerRelation: false

🤖 Generated with [Claude Code](https://claude.ai/code)

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

dev/seed.ts

@@ -1,6 +1,6 @@
 import type { Payload } from 'payload'
 
-import { devUser } from './helpers/credentials.js'
+import { devUser } from './helpers/credentials'
 
 export const seed = async (payload: Payload) => {
   // Seed default user first