Add server-source selector (same-origin default) and drop SIMTRACE_EMBEDDED

- Settings: 'Server' selector — 'Same origin' (default) vs 'Custom URL'
  (URL input revealed only in custom mode, placeholder 127.0.0.1:8081)
- serverUrl defaults to '' (same-origin); persisted per choice
- remove SIMTRACE_EMBEDDED frontend check and the server's _EMBEDDED_MARKER
  index.html injection (no longer needed)
- i18n: Server / Same origin / Custom URL (EN/RU)

simtrace2_pysniff/server/server.py

@@ -30,7 +30,6 @@ _STATIC_MIME = {
     '.map': 'application/json',
     '.wasm': 'application/wasm',
 }
-_EMBEDDED_MARKER = "<script>window.SIMTRACE_EMBEDDED='1'</script>"
 
 
 def _content_disposition(filename):
@@ -106,10 +105,6 @@ class RequestHandler(BaseHTTPRequestHandler):
             return
         with open(fs_path, 'rb') as f:
             data = f.read()
-        if rel == 'index.html':
-            # Tell the frontend it is served by this server (same origin), so
-            # it can default to a relative API base instead of 127.0.0.1:8081.
-            data = data.replace(b'</head>', _EMBEDDED_MARKER.encode() + b'</head>', 1)
         content_type = _STATIC_MIME.get(os.path.splitext(rel)[1].lower(), 'application/octet-stream')
         self.send_response(200)
         self.send_header('Content-Type', content_type)