Scaffold global navigation aligned to routes plan

Build out the full web route map from music-selfhost-routes.md as
scaffolding (no functionality on new screens):

- Full route tree: /login, /albums/:id, /artists/:id, /playlists(+detail),
  /discover, /upload, metadata editor (single + batch), /storage/maintenance,
  /queue, nested /settings and /admin, and a 404.
- Sidebar rebuilt to the A1 spec with permission-gated Discover/Upload.
- ProtectedRoute gains requirePermission; Permission exported.
- AppShell wraps Outlet in a Suspense boundary for lazy routes.
- Reusable Placeholder + SubNav; Settings/Admin become nested layouts.
- Settings/Profile: wired language + theme selectors.
- Remove orphaned Home feature (web has no Home; / -> /library) and the
  now-unused house icon + nav.home keys.
- i18n keys (en + ru) and CSS for page-title/sub-nav.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/features/metadata-editor/MetadataEditorPage.tsx

@@ -0,0 +1,18 @@
+import { useTranslation } from 'react-i18next';
+import { Placeholder } from '../../components/common/Placeholder';
+
+interface Props {
+  /** Single-track editor vs. batch editor — both A7, same scaffold. */
+  batch?: boolean;
+}
+
+/**
+ * `/tracks/:trackId/metadata` (single) and `/metadata/batch` (bulk) — A7
+ * metadata editor with auto-enrichment / diff view. Scaffold only.
+ */
+export function MetadataEditorPage({ batch = false }: Props) {
+  const { t } = useTranslation();
+  return (
+    <Placeholder title={batch ? t('pages.metadataBatch') : t('pages.metadata')} />
+  );
+}