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/lib/queryClient.ts

@@ -0,0 +1,11 @@
+import { QueryClient } from '@tanstack/react-query'
+import { STALE_PROPERTIES } from './constants'
+
+export const queryClient = new QueryClient({
+  defaultOptions: {
+    queries: {
+      staleTime: STALE_PROPERTIES,
+      retry: 1,
+    },
+  },
+})