Implement KeybindingManager singleton class to support:
- Storing and loading keybinding configuration from ini files
- Querying whether an action's keybinding was pressed
- Runtime rebinding of actions to different keys
- Default keybinding set: C=Character, I=Inventory, S=Spellbook, K=Talents,
L=Quests, M=Minimap, Esc=Settings, Enter=Chat
This is the foundation for user-customizable keybindings. Integration with
UI controls and replacement of hard-coded ImGui::IsKeyPressed calls will
follow in subsequent improvements.
