Add Google/Microsoft SSO buttons and OAuth callback flow (#24) (#41)

## Summary
- Frontend half of ai_ml_operations/chat_backend#24
- SSO buttons on Sign In / Sign Up (shown when `/public/settings/` reports provider configured)
- New `/auth/callback/` route stores JWTs, loads account, sends new SSO users through Stripe Checkout
- Error states for denied consent, unverified/missing email, link conflict, etc.
- README note on Capacitor / native redirect URI limits

## Pair with
- Backend PR: `chat_backend` branch `feature/sso-oauth-24`

## Test plan
- [ ] `npm test -- --watchAll=false --testPathPattern="SignIn.test|SignUp.test|AuthCallback.test"`
- [ ] With backend OAuth configured, Continue with Google/Microsoft from Sign In works
- [ ] First-time SSO register lands in Stripe Checkout when `needs_checkout=1`
- [ ] Existing linked user signs in and reaches app / TOS
- [ ] Error query (`email_unverified`, `access_denied`) shows message + back linkReviewed-on: #41

llm-fe/src/App.tsx

@@ -3,6 +3,7 @@ import './App.css';
 import { Routes, Route, Outlet, Navigate } from 'react-router-dom';
 import SignIn from './llm-fe/pages/SignIn/SignIn';
 import SignUp from './llm-fe/pages/SignUp/SignUp';
+import AuthCallback from './llm-fe/pages/AuthCallback/AuthCallback';
 import BillingSuccess from './llm-fe/pages/BillingSuccess/BillingSuccess';
 import BillingCancel from './llm-fe/pages/BillingCancel/BillingCancel';
 import PasswordReset from './llm-fe/pages/PasswordReset/PasswordReset';
@@ -50,6 +51,7 @@ class App extends Component {
 
                 <Route path={"/signin/"} Component={SignIn} />
                 <Route path={"/signup/"} Component={SignUp} />
+                <Route path={"/auth/callback/"} Component={AuthCallback} />
                 <Route path={"/billing/success"} Component={BillingSuccess} />
                 <Route path={"/billing/cancel"} Component={BillingCancel} />
                 <Route path={"/password_reset/"} Component={PasswordReset} />