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/admin/panels.tsx

@@ -0,0 +1,37 @@
+import { useTranslation } from 'react-i18next';
+import { Window } from '@olly/modern-sk';
+
+function StubPanel({ title }: { title: string }) {
+  const { t } = useTranslation();
+  return (
+    <Window title={title}>
+      <p style={{ color: 'var(--color-text-2)', margin: 0 }}>
+        {t('common.comingSoon')}
+      </p>
+    </Window>
+  );
+}
+
+/** `/admin/users` — user list (add/remove). Scaffold. */
+export function AdminUsers() {
+  const { t } = useTranslation();
+  return <StubPanel title={t('admin.tabs.users')} />;
+}
+
+/** `/admin/users/:userId` — per-user permissions / reset password / status. Scaffold. */
+export function AdminUserDetail() {
+  const { t } = useTranslation();
+  return <StubPanel title={t('admin.userDetail')} />;
+}
+
+/** `/admin/sources` — pluggable source management (creds/cookies/status). Scaffold. */
+export function AdminSources() {
+  const { t } = useTranslation();
+  return <StubPanel title={t('admin.tabs.sources')} />;
+}
+
+/** `/admin/instance` — service health, ML_SERVICE_URL, reindex. Scaffold. */
+export function AdminInstance() {
+  const { t } = useTranslation();
+  return <StubPanel title={t('admin.tabs.instance')} />;
+}