Drop the Shopping list count when a Supply run refetch fails (Feature 17)

The Plan page read run.data?.needed_count, which TanStack Query keeps
after a later refetch fails, so the link showed a stale count beside the
error banner. Show the count only on a successful read, and test a
successful read followed by a failed refetch.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/plan/PlanPage.test.tsx

@@ -795,6 +795,33 @@ describe('PlanPage', () => {
     expect(await screen.findByText('Opened /plan/shopping?week=2026-10-05')).toBeInTheDocument()
   })
 
+  it("drops the count when reading the week's shopping list again fails, showing the failure with Retry", async () => {
+    let failing = false
+    stubServer({
+      [`GET ${THIS_WEEK}`]: () => jsonResponse(200, planBody('draft')),
+      'GET /api/v1/shopping/2026-10-05': () => (failing ? errorResponse(500, 'internal') : jsonResponse(200, supplyRun(19))),
+    })
+    await renderPage()
+    expect(await screen.findByRole('link', { name: 'Shopping list · 19' })).toBeInTheDocument()
+
+    // Coming back to the page once the Supply run is stale reads it again,
+    // and that read fails.
+    failing = true
+    vi.setSystemTime(new Date('2026-10-07T16:01:00Z'))
+    act(() => {
+      window.dispatchEvent(new Event('visibilitychange'))
+    })
+    const alert = await screen.findByRole('alert')
+    expect(alert).toHaveTextContent("Couldn't load this week's shopping list. internal message")
+    expect(screen.getByRole('link', { name: 'Shopping list' })).toHaveAttribute('href', '/plan/shopping?week=2026-10-05')
+    expect(screen.queryByRole('link', { name: 'Shopping list · 19' })).not.toBeInTheDocument()
+    // Retry reads it again, and the count is back.
+    failing = false
+    fireEvent.click(within(alert).getByRole('button', { name: 'Retry' }))
+    expect(await screen.findByRole('link', { name: 'Shopping list · 19' })).toBeInTheDocument()
+    expect(screen.queryByRole('alert')).not.toBeInTheDocument()
+  })
+
   it('links to the Supply run with no count while the week has no shopping list', async () => {
     const fetchMock = stubServer({ [`GET ${THIS_WEEK}`]: () => jsonResponse(200, planBody('draft')) })
     await renderPage()

web/src/plan/PlanPage.tsx

@@ -378,8 +378,11 @@ function WeekBoard({
   )
   // A failed read of the week's Supply run is shown, with Retry, after the
   // Draft board's Shopping list link or under the plan manager's board; the
-  // Shopping list action still opens the Supply run.
-  const shoppingList = <ShoppingList weekStart={weekStart} neededCount={run.data?.needed_count} layout={layout} />
+  // Shopping list action still opens the Supply run. The count is that of a
+  // successful read only: a failed refetch keeps the earlier Supply run in
+  // run.data, and its count is then no longer current.
+  const neededCount = run.isSuccess ? run.data?.needed_count : undefined
+  const shoppingList = <ShoppingList weekStart={weekStart} neededCount={neededCount} layout={layout} />
   const supplyRunFailure = run.isError && (
     <ErrorBanner onRetry={() => void run.refetch()}>Couldn't load this week's shopping list. {run.error.message}</ErrorBanner>
   )