server: send Access-Control-Allow-Private-Network on CORS responses

Lets a public-hosted PWA reach a local simtrace2-pysniff-server
(http://127.0.0.1:8081); without it the browser blocks the preflight
before any request is sent.

simtrace2_pysniff/server/server.py

@@ -36,6 +36,7 @@ class RequestHandler(BaseHTTPRequestHandler):
         self.send_header('Access-Control-Allow-Origin', '*')
         self.send_header('Access-Control-Allow-Methods', 'GET, POST, PATCH, DELETE, OPTIONS')
         self.send_header('Access-Control-Allow-Headers', 'Content-Type')
+        self.send_header('Access-Control-Allow-Private-Network', 'true')
         self.end_headers()
         self.wfile.write(body)
 
@@ -45,6 +46,7 @@ class RequestHandler(BaseHTTPRequestHandler):
         self.send_header('Content-Length', len(data))
         self.send_header('Content-Disposition', _content_disposition(filename))
         self.send_header('Access-Control-Allow-Origin', '*')
+        self.send_header('Access-Control-Allow-Private-Network', 'true')
         self.end_headers()
         self.wfile.write(data)
 
@@ -74,6 +76,7 @@ class RequestHandler(BaseHTTPRequestHandler):
         self.send_header('Access-Control-Allow-Origin', '*')
         self.send_header('Access-Control-Allow-Methods', 'GET, POST, PATCH, DELETE, OPTIONS')
         self.send_header('Access-Control-Allow-Headers', 'Content-Type')
+        self.send_header('Access-Control-Allow-Private-Network', 'true')
         self.end_headers()
 
     # --- GET ---