Initial commit: shared invoice/correction-invoice PDF generation

Consolidates code previously hand-duplicated between the einfach-produktiv
frontend and payload backend repos into one canonical implementation,
consumed as a git dependency by both instead of being kept in sync by eye.

While merging the two correction-invoice copies, found and fixed three
real drifts between them:
- variantName was silently dropped in the backend's emailed Stornorechnung/
  Gutschrift, but present in the frontend's re-download copy
- the backend's correction-invoice footer wasn't position:fixed, unlike
  the original invoice and the frontend's copy
- the backend used a numeric date format (03.07.2025) while the original
  invoice and the frontend's copy both used a spelled-out month (03. Juli
  2025) — a re-download didn't visually match what was emailed

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -0,0 +1,27 @@
+{
+  "name": "@einfach-produktiv/invoicing",
+  "version": "0.1.0",
+  "private": true,
+  "description": "Shared invoice / correction-invoice (Stornorechnung, Gutschrift) PDF generation and VAT-breakdown math, consumed as a git dependency by both the einfach-produktiv frontend and the payload backend — not published to npm.",
+  "type": "module",
+  "main": "./src/index.ts",
+  "types": "./src/index.ts",
+  "scripts": {
+    "typecheck": "tsc --noEmit",
+    "test": "vitest run",
+    "lint": "eslint ."
+  },
+  "peerDependencies": {
+    "react": "^19.0.0",
+    "@react-pdf/renderer": "^4.0.0"
+  },
+  "devDependencies": {
+    "@react-pdf/renderer": "^4.5.1",
+    "@types/node": "^20",
+    "@types/react": "^19",
+    "eslint": "^9",
+    "react": "^19.2.4",
+    "typescript": "^5",
+    "vitest": "^4.1.10"
+  }
+}