Add unit tests for frontend utilities and hooks

82 tests covering download.ts and all React Query hooks. API modules are
mocked with vi.mock; mutation tests spy on queryClient.invalidateQueries
to verify cache invalidation. Conditional queries (null id) are verified
to stay idle.

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

frontend/src/hooks/usePokemon.test.tsx

@@ -0,0 +1,93 @@
+import { QueryClientProvider } from '@tanstack/react-query'
+import { renderHook, waitFor } from '@testing-library/react'
+import { createTestQueryClient } from '../test/utils'
+import {
+  usePokemon,
+  usePokemonFamilies,
+  usePokemonEncounterLocations,
+  usePokemonEvolutionChain,
+} from './usePokemon'
+
+vi.mock('../api/pokemon')
+
+import {
+  getPokemon,
+  fetchPokemonFamilies,
+  fetchPokemonEncounterLocations,
+  fetchPokemonEvolutionChain,
+} from '../api/pokemon'
+
+function createWrapper() {
+  const queryClient = createTestQueryClient()
+  const wrapper = ({ children }: { children: React.ReactNode }) => (
+    <QueryClientProvider client={queryClient}>{children}</QueryClientProvider>
+  )
+  return { queryClient, wrapper }
+}
+
+describe('usePokemon', () => {
+  it('is disabled when id is null', () => {
+    const { wrapper } = createWrapper()
+    const { result } = renderHook(() => usePokemon(null), { wrapper })
+    expect(result.current.fetchStatus).toBe('idle')
+    expect(getPokemon).not.toHaveBeenCalled()
+  })
+
+  it('fetches a pokemon by id', async () => {
+    const mon = { id: 25, name: 'pikachu' }
+    vi.mocked(getPokemon).mockResolvedValue(mon as never)
+    const { wrapper } = createWrapper()
+
+    const { result } = renderHook(() => usePokemon(25), { wrapper })
+    await waitFor(() => expect(result.current.isSuccess).toBe(true))
+
+    expect(getPokemon).toHaveBeenCalledWith(25)
+    expect(result.current.data).toEqual(mon)
+  })
+})
+
+describe('usePokemonFamilies', () => {
+  it('calls fetchPokemonFamilies and returns data', async () => {
+    const families = [{ id: 1, members: [] }]
+    vi.mocked(fetchPokemonFamilies).mockResolvedValue(families as never)
+    const { wrapper } = createWrapper()
+
+    const { result } = renderHook(() => usePokemonFamilies(), { wrapper })
+    await waitFor(() => expect(result.current.isSuccess).toBe(true))
+
+    expect(fetchPokemonFamilies).toHaveBeenCalledOnce()
+    expect(result.current.data).toEqual(families)
+  })
+})
+
+describe('usePokemonEncounterLocations', () => {
+  it('is disabled when pokemonId is null', () => {
+    const { wrapper } = createWrapper()
+    const { result } = renderHook(() => usePokemonEncounterLocations(null), { wrapper })
+    expect(result.current.fetchStatus).toBe('idle')
+  })
+
+  it('fetches encounter locations for a given pokemon', async () => {
+    vi.mocked(fetchPokemonEncounterLocations).mockResolvedValue([] as never)
+    const { wrapper } = createWrapper()
+
+    renderHook(() => usePokemonEncounterLocations(25), { wrapper })
+    await waitFor(() => expect(fetchPokemonEncounterLocations).toHaveBeenCalledWith(25))
+  })
+})
+
+describe('usePokemonEvolutionChain', () => {
+  it('is disabled when pokemonId is null', () => {
+    const { wrapper } = createWrapper()
+    const { result } = renderHook(() => usePokemonEvolutionChain(null), { wrapper })
+    expect(result.current.fetchStatus).toBe('idle')
+  })
+
+  it('fetches the evolution chain for a given pokemon', async () => {
+    vi.mocked(fetchPokemonEvolutionChain).mockResolvedValue([] as never)
+    const { wrapper } = createWrapper()
+
+    renderHook(() => usePokemonEvolutionChain(4), { wrapper })
+    await waitFor(() => expect(fetchPokemonEvolutionChain).toHaveBeenCalledWith(4))
+  })
+})