feat(showcase): add wheel, swipe and keyboard navigation to the example gallery

- Wheel on the main viewer: horizontal deltas always switch examples;
  vertical deltas switch only at the modal scroll boundary, then stay in a
  sticky session (down = next, up = prev) until the pointer leaves the area
- Touch/pen horizontal swipe switches examples; the synthesized click after
  a swipe is swallowed so the media viewer does not open
- '[' / ']' switch examples while the gallery is expanded; ArrowLeft/Right
  stay reserved for model-level navigation
- Direction-aware slide transition on every switch for visual feedback
  (respects prefers-reduced-motion)

locales/ja.json

@@ -1664,8 +1664,8 @@
                 "showCount": "例を表示（{count}）",
                 "hideExamples": "例を非表示",
                 "addExamples": "例を追加",
-                "previousExample": "前の例",
-                "nextExample": "次の例",
+                "previousExample": "前の例（[）",
+                "nextExample": "次の例（]）",
                 "noExamples": "利用可能な例画像がありません",
                 "addMoreExamples": "さらに例を追加",
                 "dragDrop": "画像または動画をここにドラッグ＆ドロップ",