[Glitch] Fix image preview too dark in alt text editor dialog

Port 2b53f93e681b323d6aab7c2dec4db10a0d94a7b1 to glitch-soc

Signed-off-by: Claire <claire.github-309c@sitedethib.com>

app/javascript/flavours/glitch/styles/mastodon/components.scss

@@ -8595,6 +8595,10 @@ noscript {
   justify-content: center;
   align-items: center;
 
+  img {
+    user-select: none;
+  }
+
   &.dragging {
     cursor: grabbing;
   }
@@ -8606,7 +8610,7 @@ noscript {
     transform: translate(-50%, -50%);
     border: 2px solid var(--color-text-on-media);
     border-radius: 50%;
-    box-shadow: 0 0 0 9999em var(--color-shadow-primary);
+    box-shadow: 0 0 0 9999em rgb(from black r g b / 25%);
     pointer-events: none;
   }
 }