Serialize the request body before fetching so it never marks the network offline

A body JSON.stringify cannot serialize, such as a cyclic one, threw inside
the fetch try/catch and was recorded as a network failure, showing the
offline pill. It now fails before the fetch with its own error.

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

web/src/api/client.ts

@@ -74,14 +74,17 @@ export function createApiClient(location: ApiLocation, network: NetworkStatus):
       }
       headers[CSRF_HEADER] = csrfToken
     }
+    // Serialize before fetching so a body that cannot be sent is its own error
+    // and never counts as a network failure.
+    let json: string | undefined
+    try {
+      json = body === undefined ? undefined : JSON.stringify(body)
+    } catch (error) {
+      throw new Error(`${method} ${path}: request body is not serializable as JSON`, { cause: error })
+    }
     let response: Response
     try {
-      response = await fetch(path, {
-        method,
-        headers,
-        credentials: 'same-origin',
-        body: body === undefined ? undefined : JSON.stringify(body),
-      })
+      response = await fetch(path, { method, headers, credentials: 'same-origin', body: json })
     } catch (error) {
       // fetch rejects with a TypeError when the network fails; an abort is a
       // DOMException and says nothing about the network.

web/src/pwa/OfflinePill.test.tsx

@@ -1,7 +1,7 @@
 import { act, render, screen } from '@testing-library/react'
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
 import { ApiProvider, createApiClient, type ApiClient } from '../api/index.ts'
-import { fakeLocation, jsonResponse, stubFetch } from '../test/fetchStub.ts'
+import { fakeLocation, jsonResponse, ME, stubFetch } from '../test/fetchStub.ts'
 import { createNetworkStatus } from './networkStatus.ts'
 import { OfflinePill } from './OfflinePill.tsx'
 
@@ -80,6 +80,22 @@ describe('OfflinePill', () => {
     expect(screen.queryByRole('status')).toBeNull()
   })
 
+  it('ignores a request body that cannot be serialized', async () => {
+    const api = renderPill()
+    const fetchMock = stubFetch(jsonResponse(200, ME))
+    await act(() => api.loadMe())
+    const body: Record<string, unknown> = {}
+    body.self = body
+
+    await act(() =>
+      expect(api.apiFetch('/api/v1/plan', { method: 'PUT', body })).rejects.toThrow(
+        'PUT /api/v1/plan: request body is not serializable as JSON',
+      ),
+    )
+    expect(fetchMock).toHaveBeenCalledTimes(1)
+    expect(screen.queryByRole('status')).toBeNull()
+  })
+
   it('ignores an aborted request', async () => {
     const api = renderPill()
     vi.stubGlobal('fetch', vi.fn<typeof fetch>().mockRejectedValue(new DOMException('aborted', 'AbortError')))