Add optional ambient animated background with toggle

Three blurred color blobs (green, blue, purple) slowly drift behind
the UI for atmosphere. Toggled via cloud icon in header, persisted
in localStorage, off by default. Works with both dark and light themes.

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

server/src/wled_controller/static/locales/zh.json

@@ -5,6 +5,7 @@
   "app.connection_lost": "服务器不可达",
   "app.connection_retrying": "正在尝试重新连接…",
   "theme.toggle": "切换主题",
+  "bg.anim.toggle": "切换动态背景",
   "accent.title": "主题色",
   "accent.custom": "自定义",
   "accent.reset": "重置",