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/not-found/NotFoundPage.tsx

@@ -0,0 +1,21 @@
+import { Link } from 'react-router';
+import { useTranslation } from 'react-i18next';
+import { EmptyState } from '../../components/common/EmptyState';
+
+/** `*` — 404. Lives inside AppShell so the sidebar/player stay visible. */
+export function NotFoundPage() {
+  const { t } = useTranslation();
+  return (
+    <div style={{ padding: '1.5rem' }}>
+      <EmptyState
+        title={t('notFound.title')}
+        description={t('notFound.description')}
+      />
+      <div style={{ textAlign: 'center', marginTop: '1rem' }}>
+        <Link to="/library" style={{ color: 'var(--color-accent)' }}>
+          {t('notFound.backToLibrary')}
+        </Link>
+      </div>
+    </div>
+  );
+}