fix(#1415): Resolve port 8080 conflict between L402 and preview server

L402 server defaulted to port 8080, which conflicts with the Nexus
preview server (#1339). Both cannot run simultaneously on the same
machine without port collision.

Changes:
  - l402_server.py: default port 8080 -> 8081, configurable via
    L402_PORT environment variable
  - app.js: update L402 cost-estimate URL from :8080 to :8081

This frees port 8080 for the preview server and makes L402 port
configurable for future flexibility.

Fixes #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("http://localhost:8081/api/cost-estimate");
   }
   
   metaLayer.track(startTime);

l402_server.py

@@ -2,6 +2,7 @@
 #!/usr/bin/env python3
 from http.server import HTTPServer, BaseHTTPRequestHandler
 import json
+import os
 import secrets
 
 class L402Handler(BaseHTTPRequestHandler):
@@ -25,7 +26,9 @@ class L402Handler(BaseHTTPRequestHandler):
             self.send_response(404)
             self.end_headers()
 
-def run(server_class=HTTPServer, handler_class=L402Handler, port=8080):
+def run(server_class=HTTPServer, handler_class=L402Handler, port=None):
+    if port is None:
+        port = int(os.environ.get('L402_PORT', 8081))
     server_address = ('', port)
     httpd = server_class(server_address, handler_class)
     print(f"Starting L402 Skeleton Server on port {port}...")