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/game/game_handler.hpp

@@ -772,6 +772,9 @@ public:
         bool     extended    = false;
     };
     const std::vector<InstanceLockout>& getInstanceLockouts() const { return instanceLockouts_; }
+
+    // Boss encounter unit tracking (SMSG_UPDATE_INSTANCE_ENCOUNTER_UNIT)
+    static constexpr uint32_t kMaxEncounterSlots = 5;
     // Returns boss unit guid for the given encounter slot (0 if none)
     uint64_t getEncounterUnitGuid(uint32_t slot) const {
         return (slot < kMaxEncounterSlots) ? encounterUnitGuids_[slot] : 0;
@@ -1743,7 +1746,6 @@ private:
     std::vector<InstanceLockout> instanceLockouts_;
 
     // Instance encounter boss units (slots 0-4 from SMSG_UPDATE_INSTANCE_ENCOUNTER_UNIT)
-    static constexpr uint32_t kMaxEncounterSlots = 5;
     std::array<uint64_t, kMaxEncounterSlots> encounterUnitGuids_ = {};  // 0 = empty slot
 
     // LFG / Dungeon Finder state