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>

tsconfig.json

@@ -0,0 +1,16 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "lib": ["ES2022", "DOM"],
+    "module": "ESNext",
+    "moduleResolution": "bundler",
+    "jsx": "react-jsx",
+    "strict": true,
+    "esModuleInterop": true,
+    "skipLibCheck": true,
+    "resolveJsonModule": true,
+    "isolatedModules": true,
+    "noEmit": true
+  },
+  "include": ["src"]
+}