Add shared VIES/VAT-ID/PLZ/carrier-tracking modules (v0.3.0)

Unifies logic that was hand-duplicated (and, for PLZ, actually inconsistent) between the backend and frontend repos — both explicitly flagged this drift risk in their own code comments. vies.ts stays behind a separate ./vies subpath (server-only by convention, unlike the client-safe main barrel).

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@einfach-produktiv/invoicing",
-  "version": "0.2.9",
+  "version": "0.3.0",
   "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",
@@ -14,6 +14,10 @@
     "./einvoice": {
       "types": "./src/einvoice/index.ts",
       "default": "./src/einvoice/index.ts"
+    },
+    "./vies": {
+      "types": "./src/vies.ts",
+      "default": "./src/vies.ts"
     }
   },
   "scripts": {