style(css): drop 2 dead @keyframes (overlayOut, panelOut)

R10 S0 audit finding: both are unreferenced — overlays and side panels
unmount with no exit animation, only their `…In` counterparts are used
(0 `animate-[…]` / `animation:` consumers). Dead code, removed.

frontend/src/index.css

@@ -419,14 +419,6 @@ html[data-perf="1"] body {
     opacity: 1;
   }
 }
-@keyframes overlayOut {
-  from {
-    opacity: 1;
-  }
-  to {
-    opacity: 0;
-  }
-}
 @keyframes panelIn {
   from {
     transform: translateX(100%);
@@ -435,14 +427,6 @@ html[data-perf="1"] body {
     transform: translateX(0);
   }
 }
-@keyframes panelOut {
-  from {
-    transform: translateX(0);
-  }
-  to {
-    transform: translateX(100%);
-  }
-}
 @keyframes popIn {
   from {
     opacity: 0;