chore(frontend): Vite-Major-Upgrade auf v8 (npm audit fix)

Schließt alle 6 gemeldeten Schwachstellen (esbuild-Dev-Server-CVE,
GHSA-67mh-4wv8-2f99), betraf nur den lokalen Dev-Server, kein
Produktions-Impact. Vite 5->8, vitest 2->5, vite-plugin-pwa 0.20->1.3.
tsc/build/vitest lokal geprüft, 0 vulnerabilities.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CVgbozhYmuEhiEJHffRXCV

frontend/package.json

@@ -37,8 +37,8 @@
     "jsdom": "^25.0.1",
     "tailwindcss": "^4.3.3",
     "typescript": "^5.5.4",
-    "vite": "^5.4.6",
-    "vite-plugin-pwa": "^0.20.5",
-    "vitest": "^2.1.9"
+    "vite": "^8.2.2",
+    "vite-plugin-pwa": "^1.3.0",
+    "vitest": "^5.0.0"
   }
 }