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/components/Queue.vue

@@ -146,7 +146,7 @@ async function removeSong(index: number) {
   }
 
   &.active {
-    background: rgba(51, 94, 234, 0.1);
+    background: var(--color-primary-10);
   }
 }