feat: enhance security by implementing HttpOnly refresh tokens and in-memory access token management

- Added isomorphic-dompurify for improved XSS protection
- Refactored token storage to use in-memory management for access tokens
- Implemented server-side route handlers for OAuth token management
- Introduced security headers in next.config.js
- Removed client-side exposure of client_secret and sensitive tokens
- Updated API interceptors to handle token refresh logic
- Cleaned up cookie management for refresh tokens

lib/tokenStore.js

@@ -0,0 +1,13 @@
+let accessToken = null;
+
+export function getAccessToken() {
+  return accessToken;
+}
+
+export function setAccessToken(token) {
+  accessToken = token || null;
+}
+
+export function clearAccessToken() {
+  accessToken = null;
+}