Implementation of 3D Hard Disk Liquid and 3D Dilute Gas Visualization

For Hard Disk Liquid:

closes #1255 (closed)

• Implement 3D particle position generation for Dense3D (Percus-Yevick hard sphere) visualization, replacing the previous placeholder that threw "not implemented yet"
• The 3D positions are generated using a molecular dynamics simulation with soft repulsive potential and Velocity Verlet integration, followed by geometric overlap resolution
• Particles are placed on the layer surface with 3D structure preserved (z-shifted so the lowest particle touches z=0)

For dilute Gas:

closes #1256 (closed)

• Implement real-space 3D visualization for Dilute3D (random gas), replacing the "not implemented yet" placeholder
• New randomPositions3D function generates random x,y,z positions in a thin slab (one particle diameter thick), with z-shifted so the lowest particle sits on the layer surface
• Uses volume density to compute particle count (N = density * Lxy^2 * Lz), capped at 200 particles
• Slab thickness matches Dense3D for consistent particle counts at equal density