feat: email receipts and repair estimates

Backend:
- Server-side HTML email templates (receipt + estimate) with inline CSS
- POST /v1/transactions/:id/email-receipt with per-transaction rate limiting
- POST /v1/repair-tickets/:id/email-estimate with per-ticket rate limiting
- customerEmail field added to receipt and ticket detail responses
- Test email provider for API tests (logs instead of sending)

Frontend:
- POS payment dialog Email button enabled with inline email input
- Pre-fills customer email from linked account
- Repair ticket detail page has Email Estimate button with dialog
- Pre-fills from account email

Tests:
- 12 unit tests for email template renderers
- 8 API tests for email receipt/estimate endpoints and validation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/backend/src/services/transaction.service.ts

@@ -10,6 +10,7 @@ import {
 import { products, inventoryUnits } from '../db/schema/inventory.js'
 import { repairTickets, repairLineItems } from '../db/schema/repairs.js'
 import { companies, locations } from '../db/schema/stores.js'
+import { accounts } from '../db/schema/accounts.js'
 import { NotFoundError, ValidationError, ConflictError } from '../lib/errors.js'
 import { TaxService } from './tax.service.js'
 import type {
@@ -473,8 +474,16 @@ export const TransactionService = {
       location = loc ?? null
     }
 
+    // Resolve customer email from linked account
+    let customerEmail: string | null = null
+    if (txn.accountId) {
+      const [acct] = await db.select({ email: accounts.email }).from(accounts).where(eq(accounts.id, txn.accountId)).limit(1)
+      customerEmail = acct?.email ?? null
+    }
+
     return {
       transaction: txn,
+      customerEmail,
       company: company
         ? {
             name: company.name,