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/Icon.tsx

@@ -19,7 +19,6 @@ import {
   GearSix,
   HardDrives,
   Heart,
-  House,
   MagnifyingGlass,
   Pause,
   Play,
@@ -48,7 +47,6 @@ import {
 
 const ICONS = {
   'vinyl-record': VinylRecord,
-  house: House,
   'magnifying-glass': MagnifyingGlass,
   'arrow-circle-down': ArrowCircleDown,
   'upload-simple': UploadSimple,