feat(tray): show now-playing track in tray + i18n menu labels (#395)

* feat(tray): show now-playing track in tray icon tooltip

Mirrors the current track to the tray tooltip ("Artist – Title") on every
play/pause/track change, falling back to "Psysonic" when nothing is playing.
The cached value survives tray hide/show via a new TrayTooltip state, and
is truncated to 127 chars for Windows NOTIFYICONDATA.szTip.

Closes #383


* feat(tray): linux fallback — disabled menu item shows now-playing track

AppIndicator on Linux has no hover-tooltip API, so the tooltip command was
a silent no-op there. Adds a disabled menu entry at the top of the tray
right-click menu that mirrors the same text. Updated in lockstep with the
Win/macOS tooltip via set_tray_tooltip.

* i18n(tray): localize tray menu labels

Adds a `tray` namespace in all 8 locales (en/de/fr/nl/zh/nb/ru/es) for
Play/Pause, Next/Previous, Show/Hide, Exit, and the Linux-only
"Nothing playing" placeholder.

Rust side: TrayMenuLabels state holds the cached translations and
TrayMenuItems holds the live MenuItem handles. New set_tray_menu_labels
command pushes new strings + applies them via set_text without rebuilding
the tray icon.

Frontend pushes the labels on mount and on every i18n.on('languageChanged').

src/locales/zh.ts

@@ -1704,4 +1704,12 @@ export const zhTranslation = {
     exitEndedBody: '"{{name}}"已结束。希望你玩得开心。',
     exitOk: '好的',
   },
+  tray: {
+    playPause: '播放 / 暂停',
+    nextTrack: '下一首',
+    previousTrack: '上一首',
+    showHide: '显示 / 隐藏',
+    exitPsysonic: '退出 Psysonic',
+    nothingPlaying: '当前没有播放',
+  },
 };