test: add shared frontend test infra (factories, mocks, render)

Foundation for the frontend test consolidation (mirrors backend #267):
- src/test/factories.ts: canonical makeNode/makeEdge/makeNodeData/makeDesign
- src/test/mocks/: reusable mockSonner/mockReactFlow/makeUseCanvasStore builders
  consumed via async vi.mock dynamic-import to survive hoisting
- src/test/render.tsx: renderWithProviders wrapping Tooltip + ReactFlow providers

Migrate ScanConfigModal sonner mock as canary. No behavior change.

ha-relevant: yes

frontend/src/components/modals/__tests__/ScanConfigModal.test.tsx

@@ -9,7 +9,7 @@ vi.mock('@/api/client', () => ({
     trigger: vi.fn(),
   },
 }))
-vi.mock('sonner', () => ({ toast: { success: vi.fn(), error: vi.fn(), info: vi.fn() } }))
+vi.mock('sonner', async () => (await import('@/test/mocks')).mockSonner())
 
 import { scanApi } from '@/api/client'
 import { toast } from 'sonner'