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.

CMakeLists.txt

@@ -20,7 +20,6 @@ find_package(OpenGL REQUIRED)
 find_package(GLEW REQUIRED)
 find_package(OpenSSL REQUIRED)
 find_package(Threads REQUIRED)
-find_package(SQLite3 REQUIRED)
 find_package(ZLIB REQUIRED)
 find_package(PkgConfig REQUIRED)
 pkg_check_modules(FFMPEG REQUIRED libavformat libavcodec libswscale libavutil)
@@ -267,14 +266,6 @@ target_link_libraries(wowee PRIVATE
     ZLIB::ZLIB
 )
 
-# SQLite
-if (TARGET SQLite::SQLite3)
-    target_link_libraries(wowee PRIVATE SQLite::SQLite3)
-else()
-    target_include_directories(wowee PRIVATE ${SQLite3_INCLUDE_DIRS})
-    target_link_libraries(wowee PRIVATE ${SQLite3_LIBRARIES})
-endif()
-
 target_link_libraries(wowee PRIVATE ${FFMPEG_LIBRARIES})
 if (FFMPEG_LIBRARY_DIRS)
     target_link_directories(wowee PRIVATE ${FFMPEG_LIBRARY_DIRS})