fix: P0 stabilisation — score modifiers, logout cache clear, provider isolation, mutation error feedback

- scoreCalculator: apply dataQuality/confidence modifiers to finalScore (were computed but hardcoded to 0)
- authService: call queryClient.clear() on logout to prevent cross-session data leakage
- queryClient: extract to src/lib/queryClient.ts singleton so services can access it without circular imports
- matchSyncService: new service layer owns match-generation logic; MockupNeedProvider no longer imports other providers directly
- hooks (11 files): add onError + German toast feedback to every useMutation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main.tsx

@@ -2,22 +2,13 @@ import { StrictMode } from 'react'
 import { createRoot } from 'react-dom/client'
 import { BrowserRouter } from 'react-router'
 import { StyledEngineProvider, ThemeProvider, CssBaseline } from '@mui/material'
-import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
+import { QueryClientProvider } from '@tanstack/react-query'
 import { theme } from './lib/theme'
+import { queryClient } from './lib/queryClient'
 import { AuthProvider } from './provider/AuthProvider'
-import { STALE_PROPERTIES } from './lib/constants'
 import './index.css'
 import App from './App.tsx'
 
-const queryClient = new QueryClient({
-  defaultOptions: {
-    queries: {
-      staleTime: STALE_PROPERTIES,
-      retry: 1,
-    },
-  },
-})
-
 createRoot(document.getElementById('root')!).render(
   <StrictMode>
     <BrowserRouter>