feat(editor): remember scroll position per scene across navigation - #57
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Switching scenes via the bottom prev/next links reset scrollTop to the cached value the browser happened to have — often partway down the previous scene's content, occasionally at 0 if the new content was shorter. Going back to an earlier scene lost any in-progress position; if the author was reading scene 3, jumped to scene 5 to fix a beat, and came back, scene 3 reappeared at the top. A Map<sceneId, scrollTop> on a ref captures the offset on every scroll tick (passive listener, no re-render). When activeSceneId changes, a second effect reads the saved value and sets scrollTop — or 0 if this is the scene's first visit. The browser clamps an oversized saved value automatically when the new scene's content height is smaller. The map lives on a ref scoped to the route instance; a new upload remounts the route, so positions reset without manual cleanup. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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功能描述
切换场景（底部"上一个/下一个"或左侧场景列表）时:
之前的行为是 `scrollTop` 不动，浏览器把上一个场景的滚动值带到新场景里，半截显示新场景中部；从场景 5 回到场景 3 时之前看到哪里完全丢失。
实现思路
两个 `useRef`:
两个 `useEffect`:
React commit 顺序保证：DOM 提交 → cleanup 旧 listener → 第二个 effect 执行设 scrollTop → 第一个 effect 挂新 listener。restore 时触发的 scroll 事件落在没监听器的窗口里，不会污染 map。
Map 挂在 ref instance 上，作用域是 Editor 路由实例；上传新文件会让 Editor 路由 unmount 重新 mount，map 自动重置，无需手动清空。
测试方式
依赖与复用声明
无新增依赖。复用现有 `flex-1 overflow-auto` 主滚动容器；只新增一个 ref。
Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com