fix(cover): round page count to even before computing spine width

Amazon rounds odd page counts up to the next even number before
calculating spine thickness. 99 pages → 100 effective pages gives
0.2347 in (5.96 mm) instead of 0.2324 in (5.90 mm).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/cover.js

@@ -90,8 +90,9 @@ async function generate() {
     console.warn('Run `pnpm build` first for an accurate spine width.\n');
   }
 
-  // Compute dimensions
-  const spineWidth  = pageCount * SPINE_PER_PAGE;
+  // Amazon rounds odd page counts up to even before computing spine width
+  const effectivePageCount = pageCount % 2 === 0 ? pageCount : pageCount + 1;
+  const spineWidth  = effectivePageCount * SPINE_PER_PAGE;
   const totalWidth  = BLEED_IN + TRIM_W_IN + spineWidth + TRIM_W_IN + BLEED_IN;
   const totalHeight = BLEED_IN + TRIM_H_IN + BLEED_IN;