remove(oauth): drop browser-initiated DCR registration flow

OAuthRegisterPage and its server routes (GET /api/oauth/register/validate,
POST /api/oauth/register) are superseded by the RFC 7591 machine-to-machine
DCR endpoint (POST /oauth/register). Claude.ai and compliant MCP clients
register via RFC 7591, then go through the standard /oauth/authorize consent
screen for scope selection.

client/src/App.tsx

@@ -13,7 +13,6 @@ import AtlasPage from './pages/AtlasPage'
 import SharedTripPage from './pages/SharedTripPage'
 import InAppNotificationsPage from './pages/InAppNotificationsPage.tsx'
 import OAuthAuthorizePage from './pages/OAuthAuthorizePage'
-import OAuthRegisterPage from './pages/OAuthRegisterPage'
 import { ToastContainer } from './components/shared/Toast'
 import { TranslationProvider, useTranslation } from './i18n'
 import { authApi } from './api/client'
@@ -167,7 +166,6 @@ export default function App() {
         <Route path="/register" element={<LoginPage />} />
         {/* OAuth 2.1 consent page — intentionally outside ProtectedRoute */}
         <Route path="/oauth/authorize" element={<OAuthAuthorizePage />} />
-        <Route path="/oauth/register" element={<OAuthRegisterPage />} />
         <Route
           path="/dashboard"
           element={