Implement `InventoryTestScene` to demonstrate reactive inventory
management using signals. Includes a grid-based visualization of items
and controls to add or remove items dynamically.
- Remove redundant `GameFlowScene` and `PlaceholderEncounterScene`
- Introduce `SceneKey` enum for type-safe scene management
- Clean up unused imports and configuration references in existing
scenes
- Standardize scene navigation using `SceneKey`
Move magic numbers and style constants to src/config.ts.
Decompose InventoryWidget into DragController, ItemRenderer,
and LostItemManager for better separation of concerns. Add
createButton utility and remove unused CombatUnitWidget.
Update scenes to import from centralized config.
hypercross