Apply i18n to all components, add collapsible sidebar, playback speed control, inline title editing, and ConfirmDialog for deletions

Phase 4: i18n all 15 components from PR #19. Sidebar gains collapse/expand
toggle. Player gets playback speed selector (0.25x-2.0x) fixing bug B1
(hardcoded Chinese '正常'). SongList adds inline title editing and model
version badge. Delete actions now use ConfirmDialog instead of
window.confirm. Volume persisted to localStorage. Training nav item
deferred to Phase 7.

components/MobileDrawer.tsx

@@ -100,7 +100,7 @@ export function MobileDrawer({ isOpen, onClose, position, children, title }: Mob
 
   const content = (
     <div
-      className={`fixed inset-0 z-50 ${backdropAnimation}`}
+      className={`fixed inset-0 z-[60] ${backdropAnimation}`}
       onClick={handleBackdropClick}
       role="presentation"
     >