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.

FEATURES.md

@@ -9,7 +9,6 @@ A comprehensive overview of all implemented features in Wowee, the native C++ Wo
 - [Network & Authentication](#network--authentication)
 - [Asset Pipeline](#asset-pipeline)
 - [Audio Features](#audio-features)
-- [Single-Player Mode](#single-player-mode)
 - [Developer Tools](#developer-tools)
 
 ---
@@ -233,7 +232,6 @@ A comprehensive overview of all implemented features in Wowee, the native C++ Wo
 - ✅ Password input field (masked)
 - ✅ Server address input
 - ✅ Login button
-- ✅ Single Player button (offline mode)
 - ✅ Connection status display
 - ✅ Error message display
 
@@ -519,32 +517,6 @@ A comprehensive overview of all implemented features in Wowee, the native C++ Wo
 
 ---
 
-## Single-Player Mode
-
-### Offline Gameplay
-- ✅ Play without server connection
-- ✅ Local character creation
-- ✅ Local character selection
-- ✅ Character persistence (SQLite)
-- ✅ Settings persistence
-
-### Simulated Systems
-- ✅ Simulated combat
-- ✅ Simulated XP gain
-- ✅ Local inventory management
-- ✅ Local spell casting
-- ✅ Simulated loot (TODO)
-- ✅ Simulated quests (TODO)
-
-### Local Storage
-- ✅ SQLite database for character data
-- ✅ Character appearance and stats
-- ✅ Inventory and equipment
-- ✅ Known spells
-- ✅ Settings and preferences
-
----
-
 ## Developer Tools
 
 ### Debug HUD (F1)