feat(folder-browser): Miller columns directory navigation

New page /folders with macOS Finder-style column layout:
- getMusicDirectory / getMusicIndexes / getMusicFolders Subsonic API methods
- Root level uses getIndexes.view (music folder IDs are not getMusicDirectory IDs)
- Columns fill available width with flex: 1, min-width: 200px
- Auto-scroll to rightmost column on expand
- Cancelled-flag prevents stale state on fast navigation
- Clicking a track plays it in the context of the current column's tracks
- FolderOpen/Folder/Music icons, accent-colored selection, ellipsis truncation
- Hidden in sidebar by default (user can enable in Settings)
- i18n: EN DE FR NL NB ZH RU

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/Sidebar.tsx

@@ -9,7 +9,7 @@ import { useTranslation } from 'react-i18next';
 import {
   Disc3, Users, Music4, Radio, Settings, Heart, BarChart3, Shuffle,
   PanelLeftClose, PanelLeft, HelpCircle, Dices, AudioLines, HardDriveDownload, Tags, ListMusic, Cast,
-  ChevronDown, Check, Music2, TrendingUp,
+  ChevronDown, Check, Music2, TrendingUp, FolderOpen,
 } from 'lucide-react';
 import PsysonicLogo from './PsysonicLogo';
 import PSmallLogo from './PSmallLogo';
@@ -28,6 +28,7 @@ export const ALL_NAV_ITEMS: Record<string, { icon: React.ElementType; labelKey:
   playlists:    { icon: ListMusic,   labelKey: 'sidebar.playlists',    to: '/playlists',      section: 'library' },
   mostPlayed:   { icon: TrendingUp,  labelKey: 'sidebar.mostPlayed',   to: '/most-played',    section: 'library' },
   radio:        { icon: Cast,        labelKey: 'sidebar.radio',        to: '/radio',          section: 'library' },
+  folderBrowser: { icon: FolderOpen,  labelKey: 'sidebar.folderBrowser', to: '/folders',        section: 'library' },
   statistics:   { icon: BarChart3,   labelKey: 'sidebar.statistics',   to: '/statistics',     section: 'system'  },
   help:         { icon: HelpCircle, labelKey: 'sidebar.help',         to: '/help',          section: 'system'  },
 };