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/SongDropdownMenu.tsx

@@ -1,5 +1,6 @@
 import React, { useEffect, useRef } from 'react';
 import { Song } from '../types';
+import { useI18n } from '../context/I18nContext';
 import {
     Video,
     Edit3,
@@ -76,6 +77,7 @@ export const SongDropdownMenu: React.FC<SongDropdownMenuProps> = ({
     onUseAsReference,
     onCoverSong
 }) => {
+    const { t } = useI18n();
     const menuRef = useRef<HTMLDivElement>(null);
 
     useEffect(() => {
@@ -174,32 +176,32 @@ export const SongDropdownMenu: React.FC<SongDropdownMenuProps> = ({
             {/* Creative Actions */}
             <MenuItem
                 icon={<Video size={14} />}
-                label="Create Video"
+                label={t('createVideo')}
                 onClick={() => handleAction(onCreateVideo)}
             />
             {isOwner && (
                 <MenuItem
                     icon={<Edit3 size={14} />}
-                    label="Edit Audio"
+                    label={t('editAudio')}
                     onClick={onEditAudio ? () => handleAction(onEditAudio) : handleEditAudio}
                 />
             )}
             <MenuItem
                 icon={<Layers size={14} />}
-                label="Extract Stems"
+                label={t('extractStems')}
                 onClick={onExtractStems ? () => handleAction(onExtractStems) : handleExtractStems}
             />
             {onReusePrompt && (
                 <MenuItem
                     icon={<Repeat size={14} />}
-                    label="Reuse Prompt"
+                    label={t('reusePrompt')}
                     onClick={() => handleAction(onReusePrompt)}
                 />
             )}
             {onUseAsReference && (
                 <MenuItem
                     icon={<Layers size={14} />}
-                    label="Use as Reference"
+                    label={t('useAsReference')}
                     onClick={() => handleAction(onUseAsReference)}
                     disabled={!song.audioUrl}
                 />
@@ -207,7 +209,7 @@ export const SongDropdownMenu: React.FC<SongDropdownMenuProps> = ({
             {onCoverSong && (
                 <MenuItem
                     icon={<Layers size={14} />}
-                    label="Cover Song"
+                    label={t('coverSong')}
                     onClick={() => handleAction(onCoverSong)}
                     disabled={!song.audioUrl}
                 />
@@ -218,17 +220,17 @@ export const SongDropdownMenu: React.FC<SongDropdownMenuProps> = ({
             {/* Library Actions */}
             <MenuItem
                 icon={<ListPlus size={14} />}
-                label="Add to Playlist"
+                label={t('addToPlaylist')}
                 onClick={() => handleAction(onAddToPlaylist)}
             />
             <MenuItem
                 icon={<Download size={14} />}
-                label="Download"
+                label={t('download')}
                 onClick={onDownload ? () => handleAction(onDownload) : handleDownload}
             />
             <MenuItem
                 icon={<Share2 size={14} />}
-                label="Share"
+                label={t('share')}
                 onClick={() => handleAction(onShare)}
             />
 
@@ -238,7 +240,7 @@ export const SongDropdownMenu: React.FC<SongDropdownMenuProps> = ({
                     <MenuDivider />
                     <MenuItem
                         icon={<Trash2 size={14} />}
-                        label="Delete Song"
+                        label={t('deleteSong')}
                         onClick={() => handleAction(onDelete)}
                         danger
                     />