Rewrite playground as struct packing visualizer

Replace the single-value encode/decode playground with a struct-oriented
tool that shows how quantizing fields changes storage slot packing.

Users define a struct with typed fields, optionally quantize them, and
see the resulting slot layout side by side with the original. Includes
reorder controls, validation, summary stats with gas savings estimates,
and generated Solidity code with copy button.

Author: 0xferit