feat(sidebar): toggle to pin Now Playing to the top (#1000)

* feat(sidebar): add toggle to pin Now Playing to the top

The fixed Now Playing entry can now be pinned to the very top of the
sidebar (above the Library label) instead of sitting above the bottom
spacer. Adds a persisted `nowPlayingAtTop` setting (default off, so
existing layouts are unchanged) and a toggle in the sidebar customizer
next to the Mix-navigation split. The entry stays non-hideable and
keeps its playing indicator in both positions.

* i18n(settings): Now Playing-at-top toggle strings (9 locales)

* docs(changelog): note Now Playing top toggle (#1000)

src/locales/zh/settings.ts

@@ -370,6 +370,8 @@ export const settings = {
   advancedBadge: '高级',
   sidebarDrag: '拖动以重新排序',
   sidebarFixed: '始终显示',
+  nowPlayingTopTitle: '"正在播放"置顶',
+  nowPlayingTopDesc: '将"正在播放"固定在侧边栏顶部，而非底部。',
   randomNavSplitTitle: '拆分混音导航',
   randomNavSplitDesc: '在侧边栏中将“随机混音”、“随机专辑”和“好运混音”显示为独立条目，而非“创建混音”合并入口。',
   tabInput: '输入',