feat: add StepFun and Nous Portal provider support (#140)

- Add StepFun provider (API key auth, STEPFUN_API_KEY)
- Add Nous Portal provider with full OAuth device code flow
  (device code request → poll for token → mint agent key → save to auth.json)
- Add NousLoginModal component for OAuth UI (user code display + verification link)
- Update ProviderFormModal to handle Nous OAuth flow (hide API key fields)
- Add nous-auth backend controller and routes
- Update PROVIDER_ENV_MAP with stepfun and nous entries
- Add i18n translations for Nous OAuth in all 8 locales

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

packages/client/src/i18n/locales/en.ts

@@ -268,6 +268,13 @@ export default {
     codexOpenLink: 'Open authorization page',
     codexApproved: 'Login successful',
     codexExpired: 'Authorization expired. Please try again.',
+    nousLoginTitle: 'Nous Portal Login',
+    nousWaiting: 'Enter this code at the authorization page to complete login:',
+    nousCopyCode: 'Code copied',
+    nousOpenLink: 'Open authorization page',
+    nousApproved: 'Login successful',
+    nousDenied: 'Authorization was denied. Please try again.',
+    nousExpired: 'Authorization expired. Please try again.',
     noProviders: 'No providers found. Add a custom provider to get started.',
     builtIn: 'Built-in',
     customType: 'Custom',