feat(oauth): browser-initiated dynamic client registration (DCR)

Adds an OAuth 2.1 public client registration flow so MCP clients can
self-register via a user-facing consent page instead of requiring manual
setup in Settings.

Server:
- DB migration adds `is_public` and `created_via` columns to oauth_clients
- New GET /api/oauth/register/validate — validates DCR params, returns
  requested scopes; unauthenticated callers get loginRequired flag
- New POST /api/oauth/register — creates a public client, saves consent,
  and redirects with client_id (cookie auth required)
- `authenticateClient` / `refreshTokens` skip secret check for public
  clients (PKCE provides the security guarantee)
- `createOAuthClient` accepts options for isPublic/createdVia; public
  clients store an opaque secret hash instead of a usable secret
- `rotateOAuthClientSecret` blocked on public clients
- `isValidRedirectUri` extracted as a shared helper
- Discovery metadata now advertises registration_endpoint and auth method
  `none`; token/revoke endpoints no longer require client_secret for
  public clients

Client:
- New OAuthRegisterPage (/oauth/register) — loading → optional
  login-required gate → scope selection → done states
- New ScopeGroupPicker component — collapsible groups, indeterminate
  checkboxes, select-all per group or globally
- oauthApi.register.{validate,submit} added to api/client.ts
- apiClient exported so it can be reused outside api/client.ts
- IntegrationsTab tests fixed for new collapsible section structure
- collab_notes fallback changed from undefined to [] in MCP trip tools

client/src/App.tsx

@@ -13,6 +13,7 @@ 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'
@@ -166,6 +167,7 @@ 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={