Add whisper support and slash command chat system

- Add WHISPER to chat type dropdown with target name field
- Implement comprehensive slash command parsing for all chat channels
- Support /w, /whisper, /tell, /t for whispers (with or without target name)
- Add /s, /y, /p, /g shortcuts for say, yell, party, guild
- Auto-fill whisper target field when switching to WHISPER with player targeted
- Allow /w <message> to whisper currently targeted player
- Add helpful error messages for invalid whisper usage
- Maintain backward compatibility with existing /logout and /invite commands

include/ui/game_screen.hpp

@@ -40,8 +40,10 @@ public:
 private:
     // Chat state
     char chatInputBuffer[512] = "";
+    char whisperTargetBuffer[256] = "";
     bool chatInputActive = false;
-    int selectedChatType = 0;  // 0=SAY, 1=YELL, 2=PARTY, etc.
+    int selectedChatType = 0;  // 0=SAY, 1=YELL, 2=PARTY, 3=GUILD, 4=WHISPER
+    int lastChatType = 0;  // Track chat type changes
     bool chatInputMoveCursorToEnd = false;
 
     // UI state