Make the navigation out of the menus themselves

Variant B of the prototype (/prototype/menu-design branch): each category is a
card whose face is the first page of that menu, so the landing is built from the
restaurant's own paper instead of labels on a dark page. The row scrolls and
snaps on a phone and wraps into a grid on a desktop; the cards are drawn from
the existing tokens, not the prototype's hexes. The bar's links become filled
segments, the current one the only teal thing on screen.

Fixes the scroller's first card being clipped: a negative-margin full bleed was
fighting scroll-snap, so the row now insets its first card and snaps to the same
inset.

docs/plan.md

@@ -89,7 +89,7 @@ Protection: a `scrypt` hash (from `node:crypto`, so no native build dependency)
 
 ### Customer design
 
-The chrome is the shelf and the menus are the paper on it: each Section is a shelf row whose right edge is a sliver of that menu's first page, bleeding off the screen. The bar names the menu you are reading and opens a sheet listing the rest, which scales to any number of categories; a 2px brass line under it tracks how far through the pages you are.
+The navigation is made of the restaurant's own paper: the landing is a row of cards, each one the first page of a menu with the category name over it and the page count on a strip beneath, scrolling sideways and snapping (wrapping into a grid on a wide screen). The bar links every menu as a filled segment, the current one the only teal thing on screen, and a 2px brass line under it tracks how far through the pages you are.
 
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