feat: 大图查看器支持 iOS 长按"存储到照片"

去掉全尺寸图上的 select-none 并显式设 -webkit-touch-callout:default,使 iOS
Safari 长按图片能弹出原生「存储到照片」菜单(保存的是 current.url 全图)。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/components/messageStream/overlays/ImageLightbox.tsx

@@ -279,11 +279,12 @@ function LightboxView({
             touchStartX.current = null;
           }}
         >
+          {/* No select-none / touch-callout:none here so iOS Safari's native
+              long-press menu ("Save in Photos") works on the full-size image. */}
           <img
             src={current.url}
             alt={current.filename}
-            className="max-h-full max-w-full select-none object-contain"
-            draggable={false}
+            className="max-h-full max-w-full object-contain [-webkit-touch-callout:default]"
           />
         </div>