feat: add boat movement and riding behavior

### Feature Description
The Boat entity already exists in the codebase but currently lacks behavior.

### Expected Behavior
- Boats float on water
- Players can mount boats
- Boats move when paddling
- Boats apply buoyancy in water

### Proposed Implementation
- Implement boat buoyancy physics when on water
- Add player riding support for boats
- Handle player movement input for paddling