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/AdminPage.tsx

@@ -1,13 +1,32 @@
+import { Outlet } from 'react-router';
 import { useTranslation } from 'react-i18next';
-import { Window } from '@olly/modern-sk';
+import { SubNav, type SubNavItem } from '../../components/common/SubNav';
 
+/**
+ * `/admin` — A9 admin shell (admin-gated). Secondary nav + nested `<Outlet/>`
+ * for users/sources/instance. `/admin` redirects to `/admin/users`.
+ */
 export function AdminPage() {
   const { t } = useTranslation();
+
+  const items: SubNavItem[] = [
+    { to: '/admin/users', label: t('admin.tabs.users') },
+    { to: '/admin/sources', label: t('admin.tabs.sources') },
+    { to: '/admin/instance', label: t('admin.tabs.instance') },
+  ];
+
   return (
-    <div style={{ padding: '1.5rem' }}>
-      <Window title={t('pages.admin')}>
-        <p style={{ color: 'var(--color-text-2)' }}>{t('common.comingSoon')}</p>
-      </Window>
+    <div
+      style={{
+        padding: '1.5rem',
+        display: 'flex',
+        flexDirection: 'column',
+        gap: '1.25rem',
+      }}
+    >
+      <h1 className="page-title">{t('pages.admin')}</h1>
+      <SubNav items={items} />
+      <Outlet />
     </div>
   );
 }

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')} />;
+}