fix: add return statements after successful plan changes to prevent double response

- Added return statement after successful paid-to-free cancellation (line 12417)
- Added return statement after successful paid-to-paid plan change (line 12448)
- Prevents 'Cannot write headers after they are sent' error
- Each plan change now returns immediately with the updated account data

chat/server.js

@@ -12411,6 +12411,10 @@ async function handleAccountSettingsUpdate(req, res) {
           subscriptionId: user.dodoSubscriptionId,
           renewsAt: user.subscriptionRenewsAt 
         });
+        
+        // Return success response immediately
+        await persistUsersDb();
+        return sendJson(res, 200, { ok: true, account: await serializeAccount(user) });
       } else {
         return sendJson(res, 400, { error: 'Failed to schedule subscription cancellation' });
       }
@@ -12438,6 +12442,10 @@ async function handleAccountSettingsUpdate(req, res) {
         if (user.scheduledPlanChange) {
           delete user.scheduledPlanChange;
         }
+        
+        // Return success response immediately after plan change
+        await persistUsersDb();
+        return sendJson(res, 200, { ok: true, account: await serializeAccount(user) });
       } catch (error) {
         log('Failed to change plan via Dodo API', { userId: user.id, error: String(error) });
         return sendJson(res, 400, { error: error.message || 'Unable to change subscription plan' });