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/nb.ts

@@ -652,6 +652,7 @@ export const nbTranslation = {
     shortcutOpenFolderBrowser: 'Åpne {{folderBrowser}}',
     shortcutFullscreenPlayer: 'Fullskjermsspiller',
     shortcutNativeFullscreen: 'Naturlig fullskjerm',
+    shortcutOpenMiniPlayer: 'Åpne minispiller',
     playbackTitle: 'Avspilling',
     replayGain: 'Replay Gain',
     replayGainDesc: 'Normaliser sporvolumet ved hjelp av ReplayGain-metadata',