feat(mini-player): user-bindable keyboard shortcut to toggle mini player

Adds an 'open-mini-player' entry to the in-app keybindings (Settings →
Shortcuts), default unbound. Pressing the chord from the main window
opens the mini and minimises main; pressing it from the mini hides the
mini and restores main — both paths invoke open_mini_player which
already handles the toggle.

The mini window has its own keydown listener (same pattern as Space /
arrows for play/skip) that reads the binding from useKeybindingsStore.
Binding changes in main propagate to an open mini through the existing
storage-event sync (now also covers psysonic_keybindings) so the user
doesn't need to restart the mini after rebinding.

Localized in all 8 supported locales.

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

src/locales/zh.ts

@@ -648,6 +648,7 @@ export const zhTranslation = {
     shortcutOpenFolderBrowser: '打开{{folderBrowser}}',
     shortcutFullscreenPlayer: '全屏播放器',
     shortcutNativeFullscreen: '原生全屏',
+    shortcutOpenMiniPlayer: '打开迷你播放器',
     playbackTitle: '播放',
     replayGain: '回放增益',
     replayGainDesc: '使用 ReplayGain 元数据标准化曲目音量',