LoadingSignal 升级：换用新品牌 logo，明亮简洁蓝色旋转动画

logo 改用 apple-touch-icon，spinner 改为蓝色渐变环 + 微光晕
+ cubic-bezier 缓动曲线，整体更轻盈现代。
同时 ScanTerminalScreen 加载态接入 ScanTerminalLoadingScreen。

frontend/components/dashboard/scan-terminal/LoadingSignal.tsx

@@ -18,12 +18,9 @@ export function LoadingSignal({
       <div className="scan-loading-spinner-wrapper" aria-hidden="true">
         <div className="scan-loading-spinner-ring" />
         <img
-          src="/static/logo.png"
+          src="/apple-touch-icon.png"
           alt="PolyWeather"
           className="scan-loading-spinner-logo"
-          onError={(e) => {
-            (e.target as HTMLImageElement).src = "/logo.png";
-          }}
         />
       </div>
       <div className="scan-loading-copy-block">