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/layout/AppShell.tsx

@@ -1,7 +1,9 @@
 import { Outlet } from 'react-router';
+import { Suspense } from 'react';
 import { Sidebar } from './Sidebar';
 import { PersistentPlayer } from '../player/PersistentPlayer';
 import { QueuePanel } from '../player/QueuePanel';
+import { LoadingSkeleton } from '../common/LoadingSkeleton';
 
 export function AppShell() {
   return (
@@ -17,7 +19,15 @@ export function AppShell() {
         <Sidebar />
         <main className="app-main">
           <div className="app-screen">
-            <Outlet />
+            <Suspense
+              fallback={
+                <div style={{ padding: '2rem' }}>
+                  <LoadingSkeleton />
+                </div>
+              }
+            >
+              <Outlet />
+            </Suspense>
           </div>
         </main>
         <QueuePanel />