feat: add visual customization presets to Settings > Appearance tab

Add style presets (font + color combinations) and background effect
presets as a new Appearance tab in the settings modal. Style presets
include Default, Midnight, Ember, Arctic, Terminal, and Neon — each
with curated dark/light theme colors and Google Font pairings.
Background effects (Dot Grid, Gradient Mesh, Scanlines, Particles)
use a dedicated overlay div alongside the existing WebGL Noise Field.
All choices persist to localStorage and restore on page load.

server/src/wled_controller/static/css/all.css

@@ -14,4 +14,5 @@
 @import './tree-nav.css';
 @import './tutorials.css';
 @import './graph-editor.css';
+@import './appearance.css';
 @import './mobile.css';