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/i18n/locales/en.ts

@@ -1,9 +1,9 @@
 const en = {
   nav: {
-    home: 'Home',
     library: 'Library',
     search: 'Search & download',
     downloads: 'Downloads',
+    upload: 'Upload',
     storage: 'Storage',
     playlists: 'Playlists',
     newPlaylist: 'New playlist',
@@ -145,6 +145,39 @@ const en = {
     downloads: 'Downloads',
     search: 'Search & Download',
     storage: 'Storage',
+    login: 'Sign in',
+    artist: 'Artist',
+    playlists: 'Playlists',
+    upload: 'Upload files',
+    metadata: 'Edit metadata',
+    metadataBatch: 'Edit metadata (batch)',
+    storageMaintenance: 'Storage maintenance',
+    queue: 'Play queue',
+  },
+  settings: {
+    language: 'Language',
+    theme: 'Theme',
+    themeDark: 'Dark',
+    themeLight: 'Light',
+    tabs: {
+      profile: 'Profile',
+      playback: 'Playback',
+      scrobbling: 'Scrobbling',
+      instance: 'Instance',
+    },
+  },
+  admin: {
+    userDetail: 'User',
+    tabs: {
+      users: 'Users',
+      sources: 'Sources',
+      instance: 'Instance',
+    },
+  },
+  notFound: {
+    title: 'Page not found',
+    description: "This screen doesn't exist yet.",
+    backToLibrary: 'Back to library',
   },
 } as const;