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/styles/shell.css

@@ -723,3 +723,44 @@
   color: var(--fg-3);
   font-size: 12px;
 }
+
+/* ============================================================
+   PAGE HEADER + SECONDARY NAV (Settings, Admin)
+   ============================================================ */
+.page-title {
+  margin: 0;
+  font-family: var(--font-display, var(--font-sans));
+  font-size: 22px;
+  font-weight: 700;
+  letter-spacing: var(--track-snug);
+  color: var(--fg-1);
+}
+
+.sub-nav {
+  display: flex;
+  gap: 4px;
+  border-bottom: 1px solid var(--hair);
+}
+.sub-nav-item {
+  padding: 8px 12px;
+  font-size: 14px;
+  font-weight: 500;
+  color: var(--fg-2);
+  border-bottom: 2px solid transparent;
+  margin-bottom: -1px;
+  transition:
+    color 0.12s ease,
+    border-color 0.12s ease;
+}
+.sub-nav-item:hover {
+  color: var(--fg-1);
+}
+.sub-nav-item.active {
+  color: var(--fg-1);
+  border-bottom-color: var(--lime);
+}
+
+/* Sidebar section header that doubles as a link (Playlists) */
+.sb-sec-link.active {
+  color: var(--fg-1);
+}