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/store/keybindingsStore.ts

@@ -12,7 +12,8 @@ export type KeyAction =
   | 'toggle-queue'
   | 'open-folder-browser'
   | 'fullscreen-player'
-  | 'native-fullscreen';
+  | 'native-fullscreen'
+  | 'open-mini-player';
 
 /** Physical keys only — ignore for binding capture */
 export const MODIFIER_KEY_CODES = [
@@ -35,6 +36,7 @@ export const DEFAULT_BINDINGS: Bindings = {
   'open-folder-browser': null,
   'fullscreen-player': null,
   'native-fullscreen': 'F11',
+  'open-mini-player':  null,
 };
 
 interface KeybindingsState {