fix: replace visibility:hidden with smooth collapse for zero-tx warning

Instead of permanently reserving space with visibility:hidden, the warning
container now uses max-height + opacity transitions. Space is reserved during
the async check, then smoothly collapses to 0 if the warning isn't needed.
This reclaims ~40px of popup viewport in the common case.

src/popup/index.html

@@ -580,7 +580,14 @@
                 <div
                     id="confirm-recipient-warning"
                     class="mb-2"
-                    style="visibility: hidden"
+                    style="
+                        overflow: hidden;
+                        transition:
+                            max-height 0.3s ease,
+                            opacity 0.3s ease;
+                        max-height: 60px;
+                        opacity: 0;
+                    "
                 >
                     <div
                         class="border border-red-500 border-dashed p-2 text-xs font-bold text-red-500"