chore: ungenutzten Frontend-Code entfernen

PageSkeleton, das shadcn Toast-System (toaster/sonner/toast/use-toast)
und das sonner-Paket waren im Code nirgends importiert. Build erfolgreich
verifiziert nach Entfernung.

package-lock.json

@@ -38,7 +38,6 @@
         "react": "^19.0.0",
         "react-dom": "^19.0.0",
         "react-hook-form": "^7.71.1",
-        "sonner": "^2.0.7",
         "tailwind-merge": "^2.2.0",
         "zod": "^4.3.5"
       },
@@ -8099,16 +8098,6 @@
         "url": "https://github.com/sponsors/ljharb"
       }
     },
-    "node_modules/sonner": {
-      "version": "2.0.7",
-      "resolved": "https://registry.npmjs.org/sonner/-/sonner-2.0.7.tgz",
-      "integrity": "sha512-W6ZN4p58k8aDKA4XPcx2hpIQXBRAgyiWVkYhT7CvK6D3iAu7xjvVyhQHg2/iaKJZ1XVJ4r7XuwGL+WGEK37i9w==",
-      "license": "MIT",
-      "peerDependencies": {
-        "react": "^18.0.0 || ^19.0.0 || ^19.0.0-rc",
-        "react-dom": "^18.0.0 || ^19.0.0 || ^19.0.0-rc"
-      }
-    },
     "node_modules/source-map-js": {
       "version": "1.2.1",
       "resolved": "https://registry.npmjs.org/source-map-js/-/source-map-js-1.2.1.tgz",