feat(ui): 全站统一首页墨黑中性风格，固定亮色并移除光暗切换

- 全站紫/青/粉写死色值与 CSS 变量改为墨黑中性色板
- 移除 aurora 光斑/星点装饰，登录弹窗改纯白卡片
- theme.js 固定亮色并清除历史 dark 选择，删除前台/后台/画布切换按钮
- 模型页空状态文案对齐「保存渠道即自动同步」

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

frontend/src/views/SettingsView.vue

@@ -273,7 +273,7 @@ async function recharge() {
       <section class="lg:col-span-2 card overflow-hidden">
         <div class="p-7 md:p-8 relative">
           <div class="absolute -top-16 -right-16 w-48 h-48 rounded-full opacity-30 blur-3xl"
-               style="background: radial-gradient(circle,#8b5cf6,transparent 60%)"></div>
+               style="background: radial-gradient(circle,#27272a,transparent 60%)"></div>
           <div class="relative grid md:grid-cols-[260px_1fr] gap-8 items-start">
             <div>
               <div class="inline-grid w-10 h-10 rounded-xl bg-orange-500/15 ring-1 ring-orange-400/30 grid place-items-center text-orange-300">