fix(client): Vite 6 statt Vite 8 wegen fehlender Rolldown-Bindings

Vite 8 benötigt native @rolldown-Bindings, die npm oft nicht installiert.
Downgrade auf Vite 6 mit plugin-react 4 behebt den Dev-Server-Absturz;
start-dev.sh prüft client/node_modules vor dem Start.

Co-authored-by: Cursor <cursoragent@cursor.com>

client/package.json

@@ -31,7 +31,7 @@
     "@types/node": "^24.12.3",
     "@types/react": "^19.2.14",
     "@types/react-dom": "^19.2.3",
-    "@vitejs/plugin-react": "^6.0.1",
+    "@vitejs/plugin-react": "^4.7.0",
     "eslint": "^10.3.0",
     "eslint-plugin-react-hooks": "^7.1.1",
     "eslint-plugin-react-refresh": "^0.5.2",
@@ -40,10 +40,10 @@
     "qrcode": "^1.5.4",
     "typescript": "~6.0.2",
     "typescript-eslint": "^8.59.2",
-    "vite": "^8.0.12",
-    "vite-plugin-pwa": "^1.3.0"
+    "vite": "^6.3.5",
+    "vite-plugin-pwa": "^1.0.1"
   },
-  "optionalDependencies": {
-    "@rolldown/binding-linux-x64-gnu": "^1.0.2"
+  "engines": {
+    "node": ">=20.0.0"
   }
 }