fix: move header inside .grid to eliminate blank first print page

fix: prevent blank first page in print — break-before/after avoid

style: portrait layout — 3 columns, A3 portrait print

style: newspaper column layout, pastel section colors, larger font

style: full redesign — white bg, sans-serif, readable 11.5px font

style: redesign visual — Claude palette, colored headers, logo, larger font

feat: expand cheat sheet to 18-19/20 coverage

feat: initial Claude Code cheat sheet — v2.1+