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/locales/zh.json

@@ -316,6 +316,7 @@
     "settings.tab.general": "常规",
     "settings.tab.backup": "备份",
     "settings.tab.mqtt": "MQTT",
+    "settings.tab.appearance": "外观",
     "settings.logs.open_viewer": "打开日志查看器",
     "settings.external_url.label": "外部 URL",
     "settings.external_url.hint": "设置后，此基础 URL 将用于 webhook 链接和其他用户可见的链接，代替自动检测的本地 IP。示例：https://myserver.example.com:8080",
@@ -1740,6 +1741,25 @@
     "bulk.disable": "禁用",
     "bulk.confirm_delete.one": "删除 {count} 项？",
     "bulk.confirm_delete.other": "删除 {count} 项？",
+    "appearance.style.label": "样式预设",
+    "appearance.style.hint": "选择一个视觉主题 — 字体和配色方案一起应用。",
+    "appearance.preset.default": "默认",
+    "appearance.preset.midnight": "午夜",
+    "appearance.preset.ember": "余烬",
+    "appearance.preset.arctic": "极地",
+    "appearance.preset.terminal": "终端",
+    "appearance.preset.neon": "霓虹",
+    "appearance.preset.applied": "样式已应用",
+    "appearance.bg.label": "背景效果",
+    "appearance.bg.hint": "在界面后面添加环境背景层。",
+    "appearance.bg.none": "无",
+    "appearance.bg.noise": "噪声场",
+    "appearance.bg.grid": "点阵",
+    "appearance.bg.mesh": "渐变网格",
+    "appearance.bg.scanlines": "扫描线",
+    "appearance.bg.particles": "粒子",
+    "appearance.bg.applied": "背景效果已应用",
+
     "color_strip": {
         "notification": {
             "search_apps": "搜索通知应用…"