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/ui/game_screen.hpp

@@ -210,6 +210,7 @@ private:
     void renderMirrorTimers(game::GameHandler& gameHandler);
     void renderCombatText(game::GameHandler& gameHandler);
     void renderPartyFrames(game::GameHandler& gameHandler);
+    void renderBossFrames(game::GameHandler& gameHandler);
     void renderGroupInvitePopup(game::GameHandler& gameHandler);
     void renderDuelRequestPopup(game::GameHandler& gameHandler);
     void renderLootRollPopup(game::GameHandler& gameHandler);