feat: add TOGGLE_BAGS action and integrate inventory screen with keybinding manager

- Add TOGGLE_BAGS action to keybinding manager (B key default)
- Update inventory_screen.cpp to use keybinding manager for bag and character toggles
- Maintain consistent keybinding system across all UI windows

include/ui/keybinding_manager.hpp

@@ -17,6 +17,7 @@ public:
     enum class Action {
         TOGGLE_CHARACTER_SCREEN,
         TOGGLE_INVENTORY,
+        TOGGLE_BAGS,
         TOGGLE_SPELLBOOK,
         TOGGLE_TALENTS,
         TOGGLE_QUESTS,