fix(css): outline article images with a 1px border for light-mode visibility

`.prose img` had `border-radius: 8px` but no border, so screenshots
that themselves have a near-white background lost their boundary
against the light-mode page background and read as floating text. The
issue was reported on the cookbook footnotes guide (7 typeset-page
screenshots) but the same condition exists on:

  - /{en,ja}/cookbook/footnotes/   (7 PNGs each)
  - /{en,ja}/themes/gallery/       (9 theme capture WebPs each)
  - /{en,ja}/viewer/vivliostyle-viewer/ (UI screenshots)

Add `border: 1px solid var(--color-border)`. The token resolves to
#e5e5e5 in light mode and #404040 in dark mode, so the outline stays
subtle but visible in both schemes — no need for a media query.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ogwata

public/styles/global.css

@@ -508,11 +508,14 @@ h1, h2, h3, h4, h5, h6 {
   background: var(--color-bg-code);
 }
 
-/* 画像 */
+/* 画像
+ * スクリーンショット等の白背景画像が明モードのページ背景と境目を失わないよう、
+ * 細罫を引く。テーマトークン経由なので明/暗どちらでも適切なコントラスト。 */
 .prose img {
   max-width: 100%;
   height: auto;
   border-radius: 8px;
+  border: 1px solid var(--color-border);
 }
 
 .prose figure {