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/nl.ts

@@ -629,6 +629,7 @@ export const nlTranslation = {
     shortcutOpenFolderBrowser: '{{folderBrowser}} openen',
     shortcutFullscreenPlayer: 'Volledigschermspeler',
     shortcutNativeFullscreen: 'Systeemvolledig scherm',
+    shortcutOpenMiniPlayer: 'Mini-speler openen',
     tabSystem: 'Systeem',
     tabGeneral: 'Algemeen',
     ratingsSectionTitle: 'Beoordelingen',