feat: refresh playground examples with 2D notebook demos

[andeplane]

Summary

The Pages playground now has an always-visible Multithreading checkbox (PR #75) that runs any script on the KOKKOS build with a thread count from the dropdown. That makes the dedicated LJ Melt (KOKKOS, multithreaded) example redundant — you can just tick the box on LJ Melt.

• Remove the LJ Melt (KOKKOS, multithreaded) example (superseded by the checkbox).
• Add two self-contained 2D examples ported from the md-basics notebook tutorials:
  • Maxwell–Boltzmann distribution (2D) — every atom starts at the same speed; collisions relax it to the Maxwell–Boltzmann form (WCA potential, enforce2d).
  • Condensation quench (2D) — a hot 2D LJ vapor quenched below its condensation point via a Langevin thermostat; droplets nucleate and coarsen.

These add physics the existing 3D-only set didn't cover (2D, WCA, Langevin thermostat, a first-order phase transition).

Test plan

• CI green (full wasm build + vitest across default / KOKKOS / atomify variants).
• Open the Pages demo, confirm the two new 2D examples appear and run, and that the KOKKOS example is gone while the Multithreading checkbox still works on any example.

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