Render the 9x9 player card as 3 stacked 3x9 mini-cards mirroring a
physical Minh Tân paper sheet, with traditional separator labels:
"Minh Tân", "Loại đặc biệt", "Tấn tài tấn lộc". Pure visual change —
underlying 9x9 data, generator, click handlers, win detection all
unchanged. Sectioned render uses absolute row index (startRow + r) so
crossed-state and rowCompleteness lookups stay correct.
Adds .section-divider (cross-hatch repeating gradient) and .section-label
styles to app.css. Defines --empty-cell-bg CSS variable with a brown
default matching the paper card; the runtime override lands with the
settings feature in a follow-up commit.