fix(dsh-plugin-browserskill): stop observation overlay from flashing between frames

Hold the last ready thumbnail until the next one decodes, and swap the
img src in place instead of remounting through a placeholder.

packages/dsh-plugin-browserskill/src/client/ObservationOverlay.module.css

@@ -79,6 +79,8 @@
   overflow: hidden;
 }
 
+/* Fade only on first mount. The <img> is reused across frames (src swap,
+   no React key) so this animation must not replay on every breath. */
 .thumb {
   max-width: 100%;
   max-height: 100%;