feat(studio): give a book its first and its last page (#2317)

A new book was a cover, a spread and a back cover, so it opened straight
onto pages 2 and 3. A bound book does not: the inside of the cover is
blank, and the first thing facing the reader is a single right-hand page,
page 1. The same at the end, where a single left-hand page sits before the
back cover. Both are laid down now, empty, in a new book and in a laid-out
one, and they are fixed the way the covers are: an inner spread only ever
goes between them.

The page numbers follow. They were counted from a spread's position in the
array, which assumed the spread after the cover was the first, and three
places did that sum on their own. One function now counts along the book,
one page for a single leaf and two for a spread, so the sheet on screen and
the label in the print view and the folio on the page all say the same
number. A book with a first page starts at 1; a book from before the first
page existed has none and keeps the numbers it always had.

client/src/components/Studio/PageNumbers.tsx

@@ -1,6 +1,7 @@
 import type { BookPageSetup, BookSpread } from '@trek/shared'
 import { fontStack } from './bookFonts'
 import { folioInk } from './folioColour'
+import { isNumbered } from './bookSheets'
 
 /**
  * The folios.
@@ -17,33 +18,46 @@ import { folioInk } from './folioColour'
  * not the ones that get printed.
  *
  * The cover and the back cover carry none. A folio on a cover is a mistake in
- * every book ever bound.
+ * every book ever bound. The single first and last pages carry one each, on
+ * the side of the book they are bound on (#2317).
  */
 export function PageNumbers({
-  spread, page, spreadIndex,
+  spread, page, folios,
 }: {
   spread: BookSpread
   page: BookPageSetup
-  /** Position in the document, cover included. */
-  spreadIndex: number
+  /**
+   * The numbers this spread carries, from foliosOf: two for a spread, one for
+   * a first or last page, none for a cover. Counted by the caller, which has
+   * the whole book — a spread on its own does not know how many pages sit
+   * before it.
+   */
+  folios: readonly number[]
 }) {
   const cfg = page.pageNumbers
-  if (!cfg?.show || spread.role !== 'inner') return null
+  if (!cfg?.show || !isNumbered(spread.role) || folios.length === 0) return null
 
   /*
-   * The cover is one sheet with a front and a back, so the first inner spread
-   * opens on `startAt` — 2 by default, the page you see when you open a book
-   * whose cover is page 1. `spreadIndex` counts the cover, hence the -1.
+   * Which page of the sheet each number sits on. A spread numbers both; the
+   * first page is a right-hand leaf and the last a left-hand one, and each
+   * carries its one number on the side it actually is.
    */
-  const left = cfg.startAt + (spreadIndex - 1) * 2
-  const right = left + 1
+  const sides: Array<['left' | 'right', number]> = spread.role === 'inner'
+    ? [['left', folios[0]], ['right', folios[1]]]
+    : [[spread.role === 'first' ? 'right' : 'left', folios[0]]]
 
   const size = cfg.size
   const y = page.pageHeight - cfg.margin
 
-  /** Where the number sits on each page, given which edge it hangs from. */
+  /**
+   * Where the number sits on each page, given which edge it hangs from.
+   *
+   * A single page is one page wide, so its number sits at x 0 whichever
+   * side of the book it is bound on; the side only decides which edge is
+   * the outer one.
+   */
   const place = (side: 'left' | 'right') => {
-    const pageX = side === 'left' ? 0 : page.pageWidth
+    const pageX = side === 'left' || spread.role !== 'inner' ? 0 : page.pageWidth
     if (cfg.position === 'centre') {
       return { x: pageX, w: page.pageWidth, align: 'center' as const }
     }
@@ -58,7 +72,7 @@ export function PageNumbers({
 
   return (
     <>
-      {([['left', left], ['right', right]] as const).map(([side, number]) => {
+      {sides.map(([side, number]) => {
         const at = place(side)
         /*
          * Sampled at the middle of the number's own box, which is where the