fix: use visibility:hidden/visible instead of CSS transitions for zero-tx warning

Remove all CSS transitions, max-height changes, and opacity animations.
The warning container always reserves its space with visibility:hidden
and switches to visibility:visible when needed. No layout shift ever.

src/popup/index.html

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