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)

include/network/net_platform.hpp

@@ -91,6 +91,20 @@ inline bool isWouldBlock(int err) {
 #endif
 }
 
+// Returns true for errors that mean the peer closed the connection cleanly.
+// On Windows, WSAENOTCONN / WSAECONNRESET / WSAESHUTDOWN can be returned by
+// recv() when the server closes the connection, rather than returning 0.
+inline bool isConnectionClosed(int err) {
+#ifdef _WIN32
+    return err == WSAENOTCONN    ||  // socket not connected (server closed)
+           err == WSAECONNRESET  ||  // connection reset by peer
+           err == WSAESHUTDOWN   ||  // socket shut down
+           err == WSAECONNABORTED;   // connection aborted
+#else
+    return err == ENOTCONN || err == ECONNRESET;
+#endif
+}
+
 inline bool isInProgress(int err) {
 #ifdef _WIN32
     return err == WSAEWOULDBLOCK || err == WSAEALREADY;