docs: add frontend test infra README (factories, mocks, split conventions)

ha-relevant: yes

frontend/src/test/README.md

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+# Frontend test infrastructure
+
+Shared helpers so tests don't re-roll the same boilerplate. Reuse these instead
+of redefining a local `makeNode`, sonner mock, or provider wrapper per file.
+
+## `factories.ts` — fixture builders
+
+```ts
+import { makeNode, makeEdge, makeNodeData, makeDesign } from '@/test/factories'
+
+makeNode('n1', { type: 'router', ip: '10.0.0.1' })   // React Flow node
+makeNode({ data: makeNodeData({ label: 'X' }) })      // partial-node form
+makeEdge('e1', 'n1', 'n2', { type: 'wifi' })          // React Flow edge
+makeDesign({ name: 'Home' })                          // Design row
+```
+
+Prefer these over an inline `const makeNode = …`. They track the domain types in
+`@/types`, so a type change surfaces in one place.
+
+## `mocks/` — reusable mock builders
+
+Consume from an **async** `vi.mock` factory via dynamic import. `vi.mock`
+factories are hoisted above the file's imports, so a plain top-level
+`import { mockSonner }` referenced inside the factory throws "cannot access
+before initialization". The dynamic `import()` resolves lazily and sidesteps it:
+
+```ts
+vi.mock('sonner', async () => (await import('@/test/mocks')).mockSonner())
+vi.mock('@xyflow/react', async () => (await import('@/test/mocks')).mockReactFlow())
+
+import { toast } from 'sonner'
+// …assert vi.mocked(toast.success) etc.
+```
+
+- `mockSonner()` — full `toast` stub (success/error/info/warning/…).
+- `mockReactFlow(extra?)` — the `@xyflow/react` exports most node/edge tests
+  need; pass `extra` to add or override an export.
+- `makeUseCanvasStore(state)` — a `useCanvasStore` replacement for component
+  tests that read a selected slice.
+
+## `render.tsx` — provider-wrapped render
+
+```ts
+import { renderWithProviders, screen } from '@/test/render'
+renderWithProviders(<NodeDetail … />)   // wraps TooltipProvider + ReactFlowProvider
+```
+
+Use the plain `render` from `@testing-library/react` for presentational
+components that need no context.
+
+## Conventions
+
+- **Store tests are split by concern** under `stores/__tests__/canvasStore/`
+  (nodes, containers, sizing, edges, selection, grouping, history, clipboard,
+  customStyle, floorMap). Add new store tests to the matching file; don't grow a
+  monolith.
+- When a component test full-mocks a store (`vi.mock('@/stores/canvasStore')`),
+  still assert the **state-driven UI branches** (disabled buttons, indicators) —
+  don't let the mock hide that wiring.