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/services/authService.ts

@@ -4,6 +4,7 @@ import type { ItemResponse } from './types'
 import { UserRole, WorkspaceType } from '../domain/enums'
 import { getPermissions, getAccessibleWorkspaces } from '../lib/permissions'
 import type { Permission } from '../lib/permissions'
+import { queryClient } from '../lib/queryClient'
 
 // Mock organizations for org switching
 const MOCK_ORGANIZATIONS: { id: string; name: string }[] = [
@@ -99,6 +100,9 @@ export const authService = {
 
   async logout(): Promise<ItemResponse<void>> {
     useSessionStore.getState().logout()
+    // Clear all cached query data so stale data from the previous session
+    // cannot leak to the next user who logs in on the same browser tab.
+    queryClient.clear()
     return { data: undefined }
   },