Validate PLZ/Packstationnummer/Postnummer format at checkout, pick up invoice layout fix

Native HTML5 pattern validation, mirroring the same rules the backend now enforces (Orders.ts/Customers.ts, same commit on that repo): PLZ digit count by country (5/DE, 4/AT+CH), Packstationnummer 1-3 digits, Postnummer 6-10 digits — catches the exact mix-up (long number in Packstationnummer, short number in Postnummer) found in a real order's data while investigating this. Client-side only for instant feedback; the backend re-validates regardless.

Also bumps @einfach-produktiv/invoicing to pick up the invoice PDF layout fixes (centered footer, clustered summary rows, relocated paid badge, stacked Packstation/Postnummer address lines).

package-lock.json

@@ -359,7 +359,7 @@
     },
     "node_modules/@einfach-produktiv/invoicing": {
       "version": "0.1.0",
-      "resolved": "git+https://git.mk360.de/Marco/einfach-produktiv-invoicing.git#f6d8e9eaf55e3a6ce3ca4b8246d5a8ed3dac7d02",
+      "resolved": "git+https://git.mk360.de/Marco/einfach-produktiv-invoicing.git#6af96eb9b8adb3caf3fb610a37c1aeb788eca2f2",
       "dependencies": {
         "@e-invoice-eu/core": "^3.1.1"
       },