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/upload/UploadPage.tsx

@@ -0,0 +1,8 @@
+import { useTranslation } from 'react-i18next';
+import { Placeholder } from '../../components/common/Placeholder';
+
+/** `/upload` — A8 drag-and-drop upload of own files. Scaffold only. */
+export function UploadPage() {
+  const { t } = useTranslation();
+  return <Placeholder title={t('pages.upload')} />;
+}