fix: resolve port 8080 conflict between preview server and L402 endpoint

## Summary
Fixed port 8080 conflict between preview server and L402 cost-estimate endpoint.

## Problem
The preview server (nginx) runs on port 8080, and the L402 cost-estimate
endpoint was hardcoded to use `http://localhost:8080/api/cost-estimate`.
This caused a conflict if both ran on the same machine.

## Solution
Updated L402 endpoint to use dynamic URL based on current host:
```javascript
`${window.location.protocol}//${window.location.host}/api/cost-estimate`
```

This ensures the L402 endpoint uses the same host as the application,
avoiding port conflicts regardless of deployment method.

## Changes
- Updated L402 cost-estimate endpoint to use dynamic URL
- No changes to L402 client logic
- Works with all deployment methods (direct, nginx, preview)

## Testing
- Verified L402 endpoint uses correct host in different deployment scenarios
- Confirmed no port conflicts with preview server
- L402 cost tracking still works correctly

Issue: #1415

app.js

@@ -681,7 +681,7 @@ function updateGOFAI(delta, elapsed) {
     
     // Simulate calibration update
     calibrator.update({ input_tokens: 100, complexity_score: 0.5 }, 0.06);
-    if (Math.random() > 0.95) l402Client.fetchWithL402("http://localhost:8080/api/cost-estimate");
+    if (Math.random() > 0.95) l402Client.fetchWithL402(`${window.location.protocol}//${window.location.host}/api/cost-estimate`);
   }
   
   metaLayer.track(startTime);