Fix Windows socket WSAENOTCONN disconnect; add boss encounter frames

Socket fixes (fixes Windows-only connection failure):
- WorldSocket::connect() now waits for non-blocking connect to complete with
  select() before returning, preventing WSAENOTCONN on the first recv() call
  on Windows (Linux handles this implicitly but Windows requires writability
  poll after non-blocking connect)
- Add net::isConnectionClosed() helper: treats WSAENOTCONN/WSAECONNRESET/
  WSAESHUTDOWN/WSAECONNABORTED as graceful peer-close rather than recv errors
- Apply isConnectionClosed() in both WorldSocket and TCPSocket recv loops

UI:
- Add renderBossFrames(): displays boss unit health bars in top-right corner
  when SMSG_UPDATE_INSTANCE_ENCOUNTER_UNIT has active slots; supports
  click-to-target and color-coded health bars (red→orange→yellow as HP drops)

src/network/tcp_socket.cpp

@@ -153,6 +153,11 @@ void TCPSocket::update() {
         if (net::isWouldBlock(err)) {
             break;
         }
+        if (net::isConnectionClosed(err)) {
+            // Peer closed the connection — treat the same as recv() returning 0
+            sawClose = true;
+            break;
+        }
 
         LOG_ERROR("Receive failed: ", net::errorString(err));
         disconnect();