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/pages/Settings.tsx

@@ -2010,6 +2010,7 @@ export default function Settings() {
                 ['open-folder-browser', t('settings.shortcutOpenFolderBrowser', { folderBrowser: t('sidebar.folderBrowser') })],
                 ['fullscreen-player', t('settings.shortcutFullscreenPlayer')],
                 ['native-fullscreen', t('settings.shortcutNativeFullscreen')],
+                ['open-mini-player',  t('settings.shortcutOpenMiniPlayer')],
               ] as [KeyAction, string][]).map(([action, label]) => {
                 const bound = kb.bindings[action];
                 const isListening = listeningFor === action;