fix(editor): correct model scaling and add reliable water rendering

[braedonsaunders]

Model positioning:

• Fix real 3D models rendering too high (floating above terrain)
• Real models are grounded at y=0, so position at terrainHeight directly
• Placeholder geometries are centered, so add height/2 for them
• Update position calculations in addObject, updateObject, updateObjectScale

Water rendering:

• Create EditorWater class with simple, reliable water visualization
• Use basic MeshLambertMaterial instead of complex ThreeWaterMesh addon
• Properly detect water regions via flood-fill algorithm
• Position water planes at correct height with proper transparency
• Replace WaterMesh import with EditorWater in EditorTerrain