fix(auth): unify browser auth with MFA across all login paths

- Create shared authenticateWithBrowserMFA utility in auth-ui.ts
- Update auth.command.ts to use shared utility for tm auth login
- Update auth-guard.ts to use shared utility for parse-prd/export
- Fix oauth-service.ts to NOT call onError for MFA_REQUIRED
  (MFA requirement is a continuation, not a failure)

All login paths now use the same MFA-aware browser auth flow:
- tm auth login
- tm parse-prd (Bring it to Hamster)
- tm export

packages/tm-core/src/modules/auth/services/oauth-service.ts

@@ -114,8 +114,9 @@ export class OAuthService {
 							error
 						);
 
-			// Notify error
-			if (onError) {
+			// Only notify error for actual failures, NOT for MFA_REQUIRED
+			// MFA requirement is a continuation of the auth flow, not an error
+			if (onError && authError.code !== 'MFA_REQUIRED') {
 				onError(authError);
 			}