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/en.json

@@ -5,6 +5,7 @@
   "app.connection_lost": "Server unreachable",
   "app.connection_retrying": "Attempting to reconnect…",
   "theme.toggle": "Toggle theme",
+  "bg.anim.toggle": "Toggle ambient background",
   "accent.title": "Accent color",
   "accent.custom": "Custom",
   "accent.reset": "Reset",