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/components/common/SubNav.tsx

@@ -0,0 +1,29 @@
+import { NavLink } from 'react-router';
+
+export interface SubNavItem {
+  to: string;
+  label: string;
+  /** Match the path exactly (used for index/redirect targets). */
+  end?: boolean;
+}
+
+interface Props {
+  items: SubNavItem[];
+}
+
+function subNavClass({ isActive }: { isActive: boolean }) {
+  return isActive ? 'sub-nav-item active' : 'sub-nav-item';
+}
+
+/** Horizontal secondary navigation for screens with sub-sections (Settings, Admin). */
+export function SubNav({ items }: Props) {
+  return (
+    <nav className="sub-nav">
+      {items.map((it) => (
+        <NavLink key={it.to} to={it.to} end={it.end} className={subNavClass}>
+          {it.label}
+        </NavLink>
+      ))}
+    </nav>
+  );
+}