Remove single-player mode to focus on multiplayer

Removed all single-player/offline mode functionality:
- Removed ~2,200 lines of SQLite database code
- Removed 11 public SP methods from GameHandler
- Removed SP member variables and state flags
- Removed SP UI elements (auth screen button, game settings)
- Removed SQLite3 build dependency
- Deleted docs/single-player.md
- Updated documentation (README, FEATURES, CHANGELOG)

Files modified:
- src/game/game_handler.cpp: 2,852 lines (down from 4,921)
- include/game/game_handler.hpp: Removed SP API
- src/core/application.cpp/hpp: Removed startSinglePlayer()
- src/ui/*: Removed SP UI logic
- CMakeLists.txt: Removed SQLite3

All online multiplayer features preserved and tested.

include/ui/auth_screen.hpp

@@ -27,10 +27,6 @@ public:
      */
     void setOnSuccess(std::function<void()> callback) { onSuccess = callback; }
 
-    /**
-     * Set callback for single-player mode
-     */
-    void setOnSinglePlayer(std::function<void()> callback) { onSinglePlayer = callback; }
 
     /**
      * Check if authentication is in progress
@@ -67,7 +63,6 @@ private:
 
     // Callbacks
     std::function<void()> onSuccess;
-    std::function<void()> onSinglePlayer;
 
     /**
      * Attempt authentication