feat(sidebar): add split Mix navigation mode with separate sidebar entries

Extract ALL_NAV_ITEMS to src/config/navItems.ts and add randomMix/randomAlbums
nav items. A new toggle in Settings switches between a single "Build a Mix" hub
and separate "Random Mix" / "Random Albums" sidebar entries (randomNavMode in
authStore). Fixes reorder crash caused by hidden items being overwritten with
undefined during the merge step.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main.tsx

@@ -1,4 +1,4 @@
-import React from 'react';
+import { StrictMode } from 'react';
 import ReactDOM from 'react-dom/client';
 import App from './App';
 import './i18n';
@@ -7,7 +7,7 @@ import './styles/layout.css';
 import './styles/components.css';
 
 ReactDOM.createRoot(document.getElementById('root')!).render(
-  <React.StrictMode>
+  <StrictMode>
     <App />
-  </React.StrictMode>
+  </StrictMode>
 );