feat(web): mobile-responsive redesign with Library view and design tokens

Add mobile layout (mini player, bottom tab bar, responsive navbar), new
Library view, redesigned bot dropdown with action buttons, and centralize
colors/typography/shadows into CSS variables in variables.scss.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/router/index.ts

@@ -13,6 +13,11 @@ const router = createRouter({
       name: 'search',
       component: () => import('../views/Search.vue'),
     },
+    {
+      path: '/library',
+      name: 'library',
+      component: () => import('../views/Library.vue'),
+    },
     {
       path: '/playlist/:id',
       name: 'playlist',