fix(search,ctx): enqueue on live-search click + reposition CM with right-click (#298)

Two coupled UX fixes around the header live search and the global context
menu, born from the same testing pass.

LiveSearch / MobileSearchOverlay
- Single click on a song no longer calls playTrack(track) without a
  queue argument. The store fell back to the existing queue, didn't
  find the new track, and ended up playing something the user couldn't
  navigate or see in the queue list — the player header showed metadata
  but the queue itself didn't contain it.
- New behaviour: enqueue([track]) + "Added to queue" toast. Whatever was
  playing keeps playing; the new track lands at the bottom and is
  visible/navigable. Mobile gets the same behaviour (tap-only).
- Desktop also gets a right-click context menu on song rows for users
  who want immediate playback or different routing (Play Now, Play Next,
  Add to Playlist, etc.). The dropdown stays open while the CM is up,
  and the row picks up the .context-active highlight so the user can
  see which song the menu refers to.

ContextMenu
- Removed the transparent fullscreen backdrop (z-index 998) that
  previously caught outside clicks. It also blocked right-clicks from
  reaching elements *underneath* it — so right-clicking a different
  song row to reposition the menu hit the backdrop instead, closed the
  menu, and never opened a new one for the row the user actually
  pointed at.
- Replaced with a document-level mousedown listener (gated on
  `contextMenu.isOpen`) that closes the menu when the click lands
  outside `menuRef`. Standard outside-click pattern, doesn't occlude
  the underlying UI, and right-clicking another row now naturally
  pivots the CM to that row.

i18n: new search.addedToQueueToast key in 8 locales.

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

src/locales/zh.ts

@@ -58,6 +58,7 @@ export const zhTranslation = {
     albums: '专辑',
     songs: '歌曲',
     clearLabel: '清除搜索',
+    addedToQueueToast: '已将"{{title}}"加入队列',
     title: '搜索',
     resultsFor: '"{{query}}" 的搜索结果',
     album: '专辑',