Fix Gesamt-Provision: shop minus employee share

Gesamt-Provision represents what the shop keeps after paying the
employee, not the sum of both. Update both the frontend KPI card
calculation and the /api/provisionen total_gesamt_provision field so
they agree.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

routes/provisionen.js

@@ -138,7 +138,7 @@ router.get('/provisionen', asyncHandler(async (req, res) => {
         total_provision: totalShop.toFixed(2),
         total_shop_provision: totalShop.toFixed(2),
         total_mitarbeiter_provision: totalMitarbeiter.toFixed(2),
-        total_gesamt_provision: (totalShop + totalMitarbeiter).toFixed(2),
+        total_gesamt_provision: (totalShop - totalMitarbeiter).toFixed(2),
     });
 }));