feat: add SPAYD QR payment code to invoice PDF

Generates QR code in SVG format using the SPAYD payment standard,
matching the PHP implementation. Contains: IBAN, amount, currency,
variable symbol, constant symbol, and invoice reference.

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

package.json

@@ -41,6 +41,7 @@
     "nodemailer": "^8.0.2",
     "otpauth": "^9.5.0",
     "prisma": "^6.19.2",
+    "qrcode": "^1.5.4",
     "react": "^18.3.1",
     "react-datepicker": "^9.1.0",
     "react-dom": "^18.3.1",
@@ -54,6 +55,7 @@
     "@types/mysql": "^2.15.27",
     "@types/node": "^25.5.0",
     "@types/nodemailer": "^7.0.11",
+    "@types/qrcode": "^1.5.6",
     "@types/react": "^19.2.14",
     "@types/react-dom": "^19.2.3",
     "@types/supertest": "^7.2.0",