Add Codex OAuth provider flow for ChatGPT account sign-in (#503)

* feat: add Codex OAuth provider flow

* fix: harden Codex OAuth storage, session activation, and UI

src/utils/effort.codex.test.ts

@@ -0,0 +1,65 @@
+import { afterEach, expect, mock, test } from 'bun:test'
+
+afterEach(() => {
+  mock.restore()
+})
+
+async function importFreshEffortModule(options: {
+  provider: 'codex' | 'openai'
+  supportsCodexReasoningEffort: boolean
+}) {
+  mock.module('./model/providers.js', () => ({
+    getAPIProvider: () => options.provider,
+  }))
+  mock.module('./model/modelSupportOverrides.js', () => ({
+    get3PModelCapabilityOverride: () => undefined,
+  }))
+  mock.module('../services/api/providerConfig.js', () => ({
+    supportsCodexReasoningEffort: () => options.supportsCodexReasoningEffort,
+  }))
+
+  return import(`./effort.js?ts=${Date.now()}-${Math.random()}`)
+}
+
+test('gpt-5.4 on the ChatGPT Codex backend supports effort selection', async () => {
+  const { getAvailableEffortLevels, modelSupportsEffort } =
+    await importFreshEffortModule({
+      provider: 'codex',
+      supportsCodexReasoningEffort: true,
+    })
+
+  expect(modelSupportsEffort('gpt-5.4')).toBe(true)
+  expect(getAvailableEffortLevels('gpt-5.4')).toEqual([
+    'low',
+    'medium',
+    'high',
+    'xhigh',
+  ])
+})
+
+test('gpt-5.4 on the OpenAI provider still supports effort selection', async () => {
+  const { getAvailableEffortLevels, modelSupportsEffort } =
+    await importFreshEffortModule({
+      provider: 'openai',
+      supportsCodexReasoningEffort: true,
+    })
+
+  expect(modelSupportsEffort('gpt-5.4')).toBe(true)
+  expect(getAvailableEffortLevels('gpt-5.4')).toEqual([
+    'low',
+    'medium',
+    'high',
+    'xhigh',
+  ])
+})
+
+test('gpt-5.3-codex-spark stays without effort controls', async () => {
+  const { getAvailableEffortLevels, modelSupportsEffort } =
+    await importFreshEffortModule({
+      provider: 'codex',
+      supportsCodexReasoningEffort: false,
+    })
+
+  expect(modelSupportsEffort('gpt-5.3-codex-spark')).toBe(false)
+  expect(getAvailableEffortLevels('gpt-5.3-codex-spark')).toEqual([])
+})