Bump @einfach-produktiv/invoicing to 0.2.9, fixing /cart crash

0.2.8's fileURLToPath fix (for a Payload-backend-only Turbopack bug)
crashed every client bundle that transitively imports fonts.ts —
including this frontend's own /cart page, via CartContent.tsx's
computeTaxBreakdown import pulling in the invoicing barrel file.
0.2.9 branches on typeof window to keep both environments working.

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

package-lock.json

@@ -455,8 +455,8 @@
       }
     },
     "node_modules/@einfach-produktiv/invoicing": {
-      "version": "0.2.8",
-      "resolved": "git+https://git.mk360.de/Marco/einfach-produktiv-invoicing.git#ed96d36b54ba529a9ce4bf4a8ee94fd4c2a50d90",
+      "version": "0.2.9",
+      "resolved": "git+https://git.mk360.de/Marco/einfach-produktiv-invoicing.git#87cf1db330e6a415fbcd31b83c6c4470a73b07a9",
       "dependencies": {
         "@e-invoice-eu/core": "^3.1.1"
       },